very nice i7-2600 system.
Agree you jsut need video to have a great system. two choices.
(1) You can get up to 100w for video from your current PSU. A card like the HD7770 is only 80W and is pretty strong.
(2) You can upgrade the PSU and get whatever you like. The only limit would possibly be the card length and case heat if the card dumped heat inside instead of dumping it outside the case. Not many case fans in an HP case. This article is great. You can use the recommendations -or- go to the last page and see how different cards compare -- for example compare the hd7770 to a higher power card and see if the hassle of the PSU swap is worth it.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
note: you want to get a quality psu. they are not necessarily expensive, for example antec earthwatt line, here is a good review of a 750w unit. You probably want more like a 500w unit.
http://hardocp.com/article/2009/09/18/antec_earthwatts_750w_psu_review/9
edit "I can only spend $200 on a GFX card. Would I be better off just waiting a while for better cards to come out?" The GT 530 is really weak. Get a new video card now. Check the hierarchy chart in the first ref above. You'll be surprised how much better the 7770 @ $125 is
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161402 or how much you'll like a monster like the $160 GeForce GTX 560 and a $60-80 PSU.