First of all IMO the CPU is not a good choice. 4 cores are not useful, single core performance is important and something like C2D E8400/8500/8600 would be better. As others mentioned.
Apart from that my opinion - for you to like the results you have to understand what period of games you are targeting. No, the fact you can run 98 and XP does not meant you can run all the games from 98 and XP.
This build, IMO, has 2 limitations:
- It is too new for 98. PCI-e will create issues, too much RAM will create issues, too fast CPU may create issues, GPU will create issues with older games. In general 98 will not be a good experience on this hardware - you'll have to mess with it a bunch to even make it work and it will break compatibility with many games.
- It is too old for XP. This GPU simply will not be sufficient for many XP games from 2004-2005 and later, not with good settings/performance.
So what this will work for would be - late 98 + early XP. And IMO for that there are better platforms, which will create less compatibility issues.
You could try to do one thing - use much later GPU, like GTX750, for XP. And use a PCI card, like FX5500, for 98. That will improve compatibility in 98 somewhat and allow you to run pretty much any XP game...
AthlonXP 2200+,ECS K7VTA3 V8.0,1GB,GF FX5900XT 128MB,Audigy 2 ZS
AthlonXP 3200+,Epox EP-8RDA3I,2GB,GF 7600GT 256MB,Audigy 4
Athlon64 x2 4800+,Asus A8N32-SLI Deluxe,4GB,GF 8800GT 1GB,Audigy 4
Core2Duo E8600,ECS G31T-M3,4GB,GF GTX660 2GB,Realtek ALC662