As for Vista I would do 3 Gigs of RAM. If you want a gaming rig then use Windows XP Pro uses a lot less RAM and more stable for driver support and service packs. Below are some links to problems with Vista that alot of people are seeing. the second link has other links there to a magazine on why not to upgrade or get vista till SP1
http://www.eggxpert.com/forums/thread/34336.aspx
http://www.eggxpert.com/forums/thread/13736.aspx
10 Annoying Vista Problems
1. Lack of available drivers from ISVs causing application conflicts;
2. Lack of available drivers for existing and new peripherals and hardware components;
3. Buggy drivers;
4. New security feature often flags existing applications as suspicious and interrupts service;
5. Few applications aside from Vista and Office 2007 take advantage of user account control;
6. Customers are confused by Office 2007's ribbon-like user interface;
7. The OS automatically recommends which Vista versions is best for each PC's configuration. This will cause small businesses to download different versions of Vista;
8. Benefits of Vista and Office 2007 running together are not obvious. Partners and IT staff need training for maximum ROI;
9. Deployment isn't easy. Partners should use either Business Desktop Deployment tool, "lite" BDD no-charge tool or other tools;
10. Hardware doesn't run Windows Vista well.