nice
suggestions:
if you're gunna stick with the 9500gt (which BTW will play all source games, evenL4D) then the PSU you have is fine, but if you listen to the other replies and do get the 9800gt, then get at least a 650 watt PSU, the 420 watt one that you have will have issues running a 9800gt.
I think the 320 gig hdd is fine, mine is 120gig (110 w/XP instaled) and i still have 86.9 gigs left even after installing l4d,CS:S

w00t

, tf2, gmod (with addons), vent hamachi, anti virus, and some photos.
processor=epic pwn (intel FTW)
2 ram is fine for vista (i have a laptop w/2gig and vista and it runs fine), plus if you want more then its way cheaper to buy it from a store and install it your self.
But i'd take this PC anyday over what i got:
pentium4 2.4 ghz and geforce 8400gs 512mb, and 512 of ram...
just my thoughts, great looking build though.
<3 i buypower
-reflex