http://www.newegg.com/Product/Produc...82E16819103776
x2 4400+ $90
http://www.newegg.com/Product/Produc...82E16813131042
Asus M2n $75
http://www.newegg.com/Product/Produc...82E16814130082
evga 320mb 8800gts $280
Gskill ddr2 800 $89.00
http://www.newegg.com/Product/Produc...82E16820231098
WD Caviar 320gig $80.00
http://www.newegg.com/Product/Produc...82E16822136074
Hiper 480w Power Supply $50 (you will definitely need a new PSU, don't skimp on this, PSU is connected to everything)
http://www.newegg.com/Product/Produc...82E16817128005
Case $28
http://www.newegg.com/Product/Produc...82E16811164040
Asus Burner $30
http://www.newegg.com/Product/Produc...82E16827135145
vista home premium 32bit $111 (Oem)
http://www.newegg.com/Product/Produc...82E16832116196
or
xp pro 32bit $90 (oem)
http://www.newegg.com/Product/Produc...82E16832116056
around $835 for a vista build without rebates (so lets just assume shipping for the whole lot is $30) there are a couple of rebates there I think but not knoweing if they'll still be around or if you'll order from newegg I've left them out
If that is the pc you're building, I'd suggest Vista. Why? well you won't have driver issues firstly, as everything there will run under vista. Secondly, 8800gts drivers work better on vista (because Direct x 10 runs on vista and not XP at least for the moment). 3 you will eventually have to go anyway to vista. Unless you're running some obscure piece of software or hardware you shouldn't have any issues. 4 I find Vista runs my games better because it utilises dual cores better. (at least I think so)
(Oh PS do you need a wireless card ford internet access or do you use a hardline? budget $30 if you need one). If it needs to be exactly $800 well, I'd go for a 160gig HDD such as this
http://www.newegg.com/Product/Produc...82E16822144415 and the rebates should take care of the shipping