It really depends on what else you have attached to the PSU
However, unless you have a lot of peripherals attached, i would think the 700W would do well. Is it a modular power supply?
However, what do you use your computer for? Do you really need a HD graphics card?
Only asking those questions as there are plenty cards out there, that do a really good job for a lot cheaper. Can you supply us a link as to which exact card you are getting please?