P45 performs better than 790i, just no sli. Although 4850 is a big downgrade from gtx280. It's just not a same thing. Get 9800gx2 instead, both an upgrade from gtx280 and cost less. Factory overclocked version for $280:
http://www.newegg.com/Product/Prod [...] 6814133231
I installed this yesterday on a 965/Q6600. Best single card I own. I have a 9600GT too. Plays all the games my son plays just fine so far. He needed a DX10 card for the Vista 64 I installed for him. He was using XP/Q6600/7900GTO. He's happy. I'm happy with it so far. Runs cool too which really surprised me. $115 after rebate and free Crysis, $49. Lots of great deals out there for OP.
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