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I would not skimp on the PSU the 700w is just right. the 500w is minimum.should be a pretty quick machine
You could cut back on the PSU a little bit if you wanted too, should budget be in the equation
The 3870 would need a 500w max i would think. the below would do ya
http://www.overclockers.co.uk/showproduct.php?prodid=CA-005-CS&groupid=701&catid=123&subcat=1084

My 1900xtx crossfired / amd 0pty 185@3120 2xHD 2xDVD 4 fans + Waterpump would knock out the silverstone zeus 650w, Enermax liberty 620w, add another 2xHD to the list & Enermax 720w would shut down when running prime95.
Dont just have enough power & strain the life out of your PSU plus lower efficiency when pushed to its limits & more crash prone.
Might well of m8.But my Enermax liberty 500w handles my system just fine.But the way your talking it should not even boot most reviews all over the net will tell you the same a GOOD 500/550 watt psu will run a x2 in a normal rig.
8gb of ram wow is that needed?4gb more than you need.with them 4 sticks of ram maybe you might need the 600 watt model.
If it was me i would get 4gb save the extra cash from that.And if you got the money for a x2 and dont have a GTX and your unwilling to wait for true next gen cards you might want to check the 9800GTX out today for the same sort of money you can get the X2 for.(yes its a refresh and not what we all wanted).