If you're running from an HDD, that's likely the main cause of the slow boot. More RAM won't make Windows boot faster unless you have a lack of RAM to begin with.
The likely reason two DIMMs is recommended over four by the motherboard manual is because it's (generally) easier to stabilize less DIMMs rather than more, but... you're already past that barrier (your four DIMMs are able to be stabilized), so you'll gain nothing to change it.
If you're looking to spend money on that at all, get an SSD. The rest isn't worth further investment; it's a 12 year old platform. 16 GB was already a lot in its time and there really is no going higher than a hyper-threaded quad core on that socket. You're basically already maxed out. Except for the HDD. SSHDs are just HDDs with an SSD cache. Yes, they are faster than HDDs when looking at their median performance... but PCs are only as fast as the slowest link in the chain. During bootup, it still has to load that data from the HDD just as if it were an HDD. That's the biggest reason why it's slow.
Now if your slightly newer PC also boots from an SSHD and is faster, then that would mean the reason the older is slower is simply down to it being slower. And if that's the case (if both systems are booting from SSHDs), then I would say the best way to spend money would be an SSD for the main system, and then the second best way to spend money would be an SSD for the secondary system. SSDs are the best general performance upgrade you can make that has come out over the last two decades.
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