Conclusion

The AMD Radeon RX 6400 comes with the narrowest PCI-Express interface of all consumer graphics cards. It's just four lanes wide, a quarter of what we're used to on graphics cards, which typically have a x16 interface. It's certainly an interesting design choice by AMD—fewer lanes result in lower GPU package pin-counts, a smaller fiberglass substrate, fewer PCB traces, etc. This normally comes in handy for mobile platforms, and it will give AMD a competitive advantage over companies like NVIDIA, who don't tinker with the PCIe lane counts on their entry-mainstream GPUs.

Averaged over our 25-game-strong test suite, we find the RX 6400 lose 14% in performance when running on the PCI-Express 3.0 interface, which is still in-use on millions of older PCs. This is the case for Intel platforms older than Rocket Lake (10th generation and older). On the AMD side, PCIe 3.0 is the fastest option when using first-generation Zen processors or lower-end motherboards with cheaper chipsets.

While 14% on average might not sound like much, it will be a big deal when looking at individual game results. For example, Assassin's Creed Valhalla and God of War lose 35% in performance, and DOOM Eternal is down 30%. On the other hand, there are titles that barely run any different—mostly older games. I think the biggest challenge for the RX 6400 on PCI-Express 3.0 is that the limited PCIe bandwidth further amplifies the effect of the card running out of VRAM.

When VRAM is tight, the operating system will automagically move resources out of graphics memory into main memory to free up space for other, more immediately needed textures and geometry. With DirectX 12 and Vulkan, this memory management can also the tuned by the game developer. The path this data takes leads across the PCI-Express bus, which means it will take longer to transfer all the bits if less bandwidth is available, so the GPU will have to stop rendering until the data it needs has finished copying to VRAM. More generally speaking, these stalls are almost always the reason why you get in-game stuttering, as the GPU is waiting on some resource before it can continue generating output.

Even when there is no VRAM pressure, reduced PCI-Express bandwidth may affect the gaming experience. All games make important decisions on the CPU: what to render, how to animate the world, where to move the player. For every frame, these instructions have to travel from the CPU to the GPU. If there's a bottleneck in that connection, latencies will go up, and the GPU will end up waiting for input data yet again, resulting in lower frame rates even when there's no memory pressure at all.

Bottom line, if you plan on running the Radeon RX 6400 on a PCI-Express 3.0 system, you should probably consider alternatives. Our recommendation for this specific scenario is a used Radeon RX 570 4 GB. It's very affordable right now at $150, and the small VRAM size makes it uninteresting to miners. Unlike the RX 6400, the RX 570 comes with a full PCIe 3.0 x16 interface, which makes a huge difference in many games—a 17% improvement overall. A very important capability these days is support for AMD Fidelity FX Super Resolution (FSR), which lets you upscale the image in supported games to achieve better performance with only a minimal performance loss. FSR is supported all the way back to the Radeon RX 400 Series, which means the RX 570 supports it, too. While FSR adoption progress has been fantastic, it's still unavailable in a vast majority of games, so usefulness is somewhat limited. AMD recently released Radeon Super Resolution; in my opinion, it is even more important than FSR because RSR works on all games, and our review confirms that it achieves good image quality. Unfortunately, RSR is only available on RDNA1 and newer, which the Radeon RX 5000 Series and RX 6000 Series utilize. Radeon RX 5000 cards are too expensive right now, though. A used RX 5500 XT will set you back $240, and the RX 5600 XT offers even worse value at $400.

NVIDIA's counterpart to AMD RSR is NVIDIA Image Scaling (NIS), which is supported on all cards since Pascal. This means the GTX 1060 6 GB could be an option if you find it at an attractive price, below $200, I'd say. The GTX 1650 and 1650 Ti might also be interesting as long as they are reasonably priced; check our Performance per Dollar chart on the previous page to get a feel for the price points we used and how they affect a card's value. Neither of those cards support DLSS, however, which is NVIDIA's AI powered upscaling technology. For DLSS, you'll need an RTX 2060 instead, which is probably too expensive at $330.