Quick: Windows Vista, XP and 4 Gb of RAM and performance

Best explanations:
http://makfu.wordpress.com/2007/08/09/okay-one-more-time/

More here:
4sysops.com: Why Windows Vista only sees 3Gb [of] memory in a PC with 4Gb [of] RAM and how [Windows] Vista SP1 fools its users

What to tweak and what not to tweak on Windows XP/Vista:
Lifehacker: Debunking common Windows performance tweaking myths