Need to set expectations...

[Ominous4]

I have a HP Z620 workstation that is still serving all my purposes just fine. It supports PCIe 3.0 x16. I currently run a R9 Nano video card which maxes that out. I am looking at getting a used RX 6600 XT, which runs at PCIe 4.0 x8. Supposedly, this card was designed to physically only support eight lanes because eight lanes on Gen 4.0 is basically the same as sixteen lanes on Gen 3.0. Well, given that my motherboard only supports Gen 3.0, I think this means the "new to me" card would only actually support PCIe 3.0 x8 which is half of what my R9 Nano uses. What I need help with is (1) making sure I am accurate in this assessment and (2) understanding the potential ramifications from a performance perspective. For example, the "new to me" card would have double the RAM and be a lot faster (approximately twice the speed, if using Gen 4.0 x8). However, how would it potentially perform given that it would be limited to Gen 3.0 x8 instead of Gen 4.0 x8?

 

[AtiAmd]

If you dont care about power consumption get rtx 2070 super or 2080 but the thing is... pcie 3.0 means old cpu too so why not get 1660ti coz your cpu will bottleneck higher card.

 

[DarkDreams]

You might want to read TPUs article on exactly that topic. Short version is that the performance of the card isn't high enough to saturate even 3.0x8 in almost all cases:

 

[Palindrome]

Absolutely check out the article @DarkDreams linked, for the most part the differences in performance between 3.0 and 4.0 are effectively neglible. I used to run my 6600 XT in a PCIe 3.0 system, and it did run at PCIe 3.0 x8, as you correctly assessed. I found an old screenshot from GPU-Z that I took:

As you can see, GPU-Z lists it as "PCIe x8 4.0 @ x8 3.0". I cannot say I noticed any performance issues that I would've tallied up to a lack of PCIe bandwidth.

If anything, a more pressing concern may be whether your HP motherboard will accept a RDNA 2 card. I've had times in the past where Dell and HP prebuilts would only accept GPUs from around their own age, anything too new and they simply wouldn't recognize the device. That of course depends on the age/generation of your HP workstation.

 

[PLAfiller]

As others mentioned I wouldn't worry about PCIe 4.0 card into PCIe 3.0 slot.

My mobo is PCIe 3.0 and I have RTX A2000, which is a PCie 4.0 card.....zero issues with it.

What you should really check out is: 1.) whether the PSU in your workstation can handle the new card and 2.) as @Palindrome said, whether your BIOS/ MB will accept the new card.

I don't think you will have problems with the RX6600, it's TDP is 132W and your current card is rated @175W, so you can handle the new card.

When new, this machine was sold with the following GPUs:

Professional 2D: NVIDIA NVS 300, NVIDIA NVS 310,** NVIDIA Quadro NVS 450, AMD FireProTM 2270 Entry 3D: NVIDIA Quadro 410,** NVIDIA Quadro 600, AMD FireProTM V3900, AMD FireProTM V4900 Mid-range 3D: NVIDIA Quadro 2000, AMD FireProTM V5900 High-end 3D: NVIDIA Quadro 4000, AMD FireProTM V7900, NVIDIA Quadro 5000, NVIDIA Quadro 6000, NVIDIA Tesla C2075

 

[Ominous4]

Thank you, everyone! That's exactly what I needed to know. That TPUs article is excellent!

@AntiAmd -- I actually had a GTX 1080 8GB that a friend gave me. It worked just fine for gaming, but I could not get it to work reliably in Linux (daily use, not even gaming). I was always dealing with stability issues. For example, whenever I put my computer to sleep (I usually have a LOT open since my computer has 20 cores and 192GB RAM), it would not wake up. I'd have to hard reboot it. I knew this would eventually result in massive file system corruption. I truly do wish NVIDIA would do better on open source drivers for Linux. I actually envy the NVIDIA/AMD/Intel competition we have in Windows and wish we had that in Linux just as much.

 

[eidairaman1]

Absolutely check out the article @DarkDreams linked, for the most part the differences in performance between 3.0 and 4.0 are effectively neglible. I used to run my 6600 XT in a PCIe 3.0 system, and it did run at PCIe 3.0 x8, as you correctly assessed. I found an old screenshot from GPU-Z that I took:
View attachment 380044
As you can see, GPU-Z lists it as "PCIe x8 4.0 @ x8 3.0". I cannot say I noticed any performance issues that I would've tallied up to a lack of PCIe bandwidth.

If anything, a more pressing concern may be whether your HP motherboard will accept a RDNA 2 card. I've had times in the past where Dell and HP prebuilts would only accept GPUs from around their own age, anything too new and they simply wouldn't recognize the device. That of course depends on the age/generation of your HP workstation.

Known as bios white listing which occurs in mobiles

As others mentioned I wouldn't worry about PCIe 4.0 card into PCIe 3.0 slot.

My mobo is PCIe 3.0 and I have RTX A2000, which is a PCie 4.0 card.....zero issues with it.

What you should really check out is: 1.) whether the PSU in your workstation can handle the new card and 2.) as @Palindrome said, whether your BIOS/ MB will accept the new card.

I don't think you will have problems with the RX6600, it's TDP is 132W and your current card is rated @175W, so you can handle the new card.

When new, this machine was sold with the following GPUs:

Professional 2D: NVIDIA NVS 300, NVIDIA NVS 310,** NVIDIA Quadro NVS 450, AMD FireProTM 2270 Entry 3D: NVIDIA Quadro 410,** NVIDIA Quadro 600, AMD FireProTM V3900, AMD FireProTM V4900 Mid-range 3D: NVIDIA Quadro 2000, AMD FireProTM V5900 High-end 3D: NVIDIA Quadro 4000, AMD FireProTM V7900, NVIDIA Quadro 5000, NVIDIA Quadro 6000, NVIDIA Tesla C2075

A system from 2011, wow...

 

[Hyderz]

Running a 3090 on my pcie3.0, I’ve seen some benchmarks and the difference is 1-3fps at most

 

[iameatingjam]

Pcie 3 on 8 lanes can actually be okay as long as there's enough onboard vram that none needs to be borrowed from dram. Unfortunately with an 8gb card, thats unlikely to always be the case, so you'd lose some performance in such a case, depends on the game how bad it would be. If I were you, I'd want something with either 16GB vram ( like 4060 ti or 7600xt) or something with 16 lanes ( but also more vram than 8gb), most cards have 16 lanes so there's a lot of options, but if you want budget and not have something wasted on a cpu (btw, what is it?).... Hmmm, used 6700/xt? Or if you want nvidia, idk, the 3060 I guess ( make sure its the one with 12GB). Both have 16 lanes and are above 8gb. Though I believe the 6700xt is more powerful, though the 3060 has dlss, so there's that.

Check this out: