How big of a diffence will there be playing games like half life 2 if i use a Athlon 64 3200+, 6600 TD TDH, 1.5 Gig Ram versus, a Athlon Xp 3000+, 6600 TD TDH, 1.5 Gig Ram?
6800GT is the best to get. You can easily overclock it to a 6800Ultra, so theres no point in spending the extra $$ to get a 6800Ultra. I suggest you keep your current setup and get a better vid card (6800GT = ~$300.)