you need a good cpu (dual core not advised), good mobo with at least two pcie x16 slots for cfx/sli, a strong psu to run two cards. vp450 is an entry level psu better suited for a single card setup.
it's not a good idea to start with entry level cfx/sli combo. better to go with single powerful card in the begining and if budget permits, add one more down the road.
iirc 7750 cfx would perform close to single 7850 (check tweaktown.com benchmarks). 7850 outperforms gtx 560.
iam getting an i5 also btw how much performance will i lose if chosen 560 over 7850 and is sparkle gtx 560 any good