Getting the i5 750 is a great choice for a CPU, so you aren't going wrong there.
Sticking with the GTX 275 isn't bad idea either. It is still a very good performing card in gaming. I'm running two GTX 260 Core 216's and don't plan on updating until the next gen GTX 4xx series cards come out. I'm not impressed with the GTX 470 & 480 when it comes performance to price.
I say stick with the i5 750, GTX 275 and go with the ASUS P7P55D-E Pro but of course it isn't my money to spend