Only the Corsair Professional and Professional Gold series (HX and AX) are modular systems.
I would go with the HX850 as first choice, the TX850 V2 (only the V2 I might add!) as second, followed by the GS800.
I would also be hitting for a 850W PSU if it was me, as your hexacore will be hitting quite high wattage overclocked, and you need the headroom if you plan to OC the HD6950's further than the factory OC's.
It is also considered unwise to fully stress any PSU for prelonged periods, and you should always build your system with capacity to spare - over time PSU's age, and the rated capacity decreases, added to which running it at max power (in the case of a underpowered system) will cause premature failing of the PSU, and is likely to affect voltage fluctuation due to excessive overloading. That also puts components elsewhere in the system at risk, so in short, don't skimp and more is better (within reasons of course)
In either case, the HX850 is a bargain deal, and even HardOCP.com agree that its top quality, giving it a Gold rating - Read the full review here:
http://hardocp.com/article/2009/05/27/corsair_hx850w_power_supply/
The TX850 fairs nearly as welll too, HardOCP.com giving it a Silver Award. Full review here:
http://hardocp.com/article/2008/12/19/corsair_tx850w_power_supply/
For clarity, HardOCP.com are highly regarded are the reviewer of choice for PSU's, and more fail than pass their rigorous tests, so the units than pass are top quality. Either choice is a wise one from a PSU standpoint.
P.S. Thanks Mizzou.