Kinda depends on the game although there are quite a few factors involved. My experience is that 7 will run games just as well as xp but vista at times was truly awful.
Do you have any particular games in mind?
Well like I said there are other factors to take into account.. For example, the xp box might have the correct drivers and be running like a peach whereas the vista/7 box could have all the wrong drivers and be running terrible... I'm not saying this is whats happening in your case but just that it's not usually as cut and dried as you think....
Perhaps the best way to approach this is to get your 7 machine running the best it possibly can and then bench..
So with that in mind, is the 7 machine running with all the latest drivers and up to date bios?
Yep new chipset drivers and all. My GPU drivers aren't "official" though but they are modded for speed. I tried stock drivers and they gave the same FPS in WoW and War3 but then CS:S and Gmod (all source games in general) were slower.
Under XP I should imagine the above works just fine but for vista or 7 (as you've found out) it's just not going to cut the mustard...
Sorry man but you really need something a bit more powerful for decent gaming.. (using either vista or 7)
It's because vista is power hungry and on your machine this will be more evident by the software you run. Don't forget that vista has a huge amount of bloatware so it needs to run that as well as any software it's given like a game. So if the power isn't there then it's going to be obvious by the frame rate..
If you had a 3GHz Dual/quad core system that was reasonably new with 2GB of RAM and upwards the games would run probably pretty similar to xp. Your system however just isn't powerfull enough to reproduce the same results..
You could try right click My Computer, Properties, a windows will appear, on the left is Advance System Settings. Click on it, System Properties window will appear, Under Advance Tab, Performance Settings\Visual Effects, Tick the radial "adjust for best performance".
Try that, that might squeeze a little bit more out for ya, set it back when your finished gaming if you like.