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Em...I don't think you know about the performance of a GTX650...it only roughly on par with the 7750, which is only slightly faster than the 8800GTS 512MB that Op already got...Yes it's a lot better than a 8800GTS.
Yes it would be faster, but in real-world term on performance it would be a bit like upgrading from a 4830 toa 4870...Not much in the way of comparisons, but it looks like it is better :
http://www.anandtech.com/bench/product/681?vs=521