Why waste money on getting two GTS?..Just nab a 8800 GTX...
You'll get better performance out of a single card. It'll end up being cheaper also. Overall though if you are buying a rig for DX10 It'd be wise not to buy any card just yet. The current cards out won't do a good job on full DX10 games...For that you'd have to wait for g92/r700. G92 should be out within this year...Towards the end.
What are you current specs?..there is so much coming out around the corner I wouldn't recommend getting a new rig if it's for DX10...penryn, barcelona, x38, g92 and r700...All coming out this year...Also if you are a big AMD fan..2.6 + models of barcelona will be this year as well...Not q1 or q2 2008 as once thought.
Well, a Corsair 620HX would power all that fine, and it's very reliable and stable. If you think 620 watts is not enough, it's rated at 620 watts at 50 degrees Celsius, while most other power supplies are rated at 40 or 30. This means that it's comparable to approximately a 700 watt power supply from another company.
Echoing others' thoughts, why go for two GTS cards? And how exactly are you planning on overclocking the 6000+ to 3.2 Ghz? What kind of cooling do you plan on having?
Wait till the middle of August and get a single 8800 GTX by EVGA and EVGA only, G92 is suppose to be out in November so you'll still be able to use the 3 months step up program and get G92 card for only $150 or so, which will be 2.5x faster.