Won't support a dual slot card. Might if you can modify the cooling of either the card or the CPU. (smaller fan, water cooling the CPU, etc.) The fastest single slot AMD card I found was a 5770.http://www.newegg.com/Product/Product.aspx?Item=N82E16814150501
$163 shipped, $143 if you get the rebate. Single slot like your 7800GT. If you insist on Nvidia, I found a 9800GT.http://www.newegg.com/Product/Product.aspx?Item=N82E16814500115
If you consider the performance difference, as long as you get the rebate the extra $20 is worth it. You might be able to get away with one of the "egg" cooler cards, something like this GTS450.http://www.newegg.com/Product/Product.aspx?Item=N82E16814125342
$114 shipped, $104 after rebate.
You might need to do some shaving on either/both coolers to get it to fit. That card looks pretty barren on the back so there shouldn't be anything pressing against the CPU. Not sure if its worth the try. Perhaps find someone local with a card like that and see?