I agree, best to build yourself.....but if you insist on getting a prebuilt I would recommend monarchcomputers.com over ibuypower.....if you insist on that system as above go with that ernemax psu or antec or ocz......get 2gb of ram for sure if your gaming and that 7800gt is great....good luck, we are here for any other help necessary...
Get an asus a8n or msi neo4 board, or even better a DFI lanparty ultra board...
BTW John3, you have a "perfect" system....hows that 7900gt workin for ya....?