I have a few different OS's and have been trying to find just the right one that fits for gaming with my latest pc upgrades. I personally like XP Pro 32 bit the best of all since almost all software vendors/drivers support it.
For the past week, I had reinstalled XP Pro x64 but once again, had a lot of driver problems as I couldn't seem to find most of them. Not many anti-virus programs seem to run on it ect. However, XP Pro x64 doesn't use much more resources than XP Pro 32 bit.
This past week, I bought Vista Ultimate 64 bit from Newegg.com, it's like $189.00 for the Vista Ultimate x64 OEM disk. I've not had any driver issues or anything. Some software is still lacking support but I don't use much. I play Call of duty 4 most of the time. Vista x64 does use more system resources than xp 64 but not by much. I used Vlite to get rid of what I didn't need and then use ccleaner to keep it uncluttered. Then I use other programs like TweakVI to help optimize it and VistaGlazz to add custom themes.
I'm running Vista Ultimate x64 on a 2 year old Dell XPS 400, Pentium D 820 (2 cores) 2GB ram, 9600 GT OC video card. And using about 22% ram (468MB) while idle. So it's about double that of XP but once you learn a few of the quirks like the UAC, it really isn't that bad. I honestly don't see much of an advantage over 32 bit XP but then again, I don't have a blazing fast system either.
Anyway, those are just my thoughts,