hello and welcome,
it's a good starter desktop, but not knowing what PSU you have is troublesome. Could you check your documentation or open the computer up and check the PSU model/markings inside?
or what is your system service tag?
my guess is you have something around 400-550W PSU. That leaves a decent range of cards available, but not knowing whether you will need to upgrade the PSU or not leaves you hanging.
Basically, for $200 you could pick up 560Ti or 6950, but that requires at least 500W PSU. So, if you need to get a PSU for $50-80, then you're left with $170 worst case, at which point you could settle for 6870 (~$185)
(see here: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-4.html)
So, first order of business, figure out exactly what PSU you have.
Also, if you could tell us exactly which games you prefer that could help a bit (random MMOs typically aren't very GPU heavy, but if you get something like witcher 2 on steam that's a significant tax on your video card if you run it on high settings)