well seeing that you have a 8800 gtx and hopefully game at a resolution of at least 1600x1200, you probably wouldn't even notice a performance hit swapping cpu's. and every oc will be different with each cpu but yes you should be able to match performance...
a setting to let your vid card run in sync with your monitors refresh rate to reduce tearing ex-vsync enable 60Hz, ur vid card will only go up to 60fps
it is cheaper because it is oem, meaning not coming in a retail box and all that. here is a good option for 250 http://www.newegg.com/Product/Product.asp?Item=N82E16814102051
i would upgrade now, quadcore is still months away. once intel quad comes then you are gonna see amd quad on the way and wait longer just get am2 right now or c2d and be happy