Ok, so here is sum serious stuff.
First of all, to the Original Poster, dude, with tat huge config in mind and in reality, PLZ go for atleast 750W of PSU...
Check it here if u wat to... --->
http://extreme.outervision.com/psucalculatorlite.jsp
See my siggy, and see the PSU tat i hv got (coz i will be adding 2 more gtx280, in total 3 x gtx280's in three way SLi)...
U better go for ATLEAST Corsair TX750, or if u hv upgradation in mind, like suppose u wanan add one more 4870 (to make it Crossfire)..go with atleast 1000 or 1100W psu from Tagan.....
Both Corsair and Tagan are best of brands..

-----------
Corsair 750W PSU (model TX750) ---------------> Rs. 7K
Tagan 1100W PSU (model: BZ1100) -------------> Rs. 14.1K
--------------
Cheers n e-peace...
Using tat paltry low brand PSU of 300-350W will jus NOT run ur CPU AT ALL... buddy...

-----------
So, here is the clarification:
1.) Jus one 4870 (and NOT planning to upgrade even in future) -------> 750W PSU (corsair)
2.) Planning to add more than one 4870 and also for futureproofing ---> 1100W PSU (tagan)