The very max card for your x2 4200+ CPU would be HD5770. If you are gaming at 1680 x 1050 or lower resolution you might get as happy with GTS250, HD4850 or HD5750 the two first ones can be had for less than $100 on newegg.com most of the time.
also i posted this in the cpu section but can i just swap a am2 6000+ into it without any other changes? i dont know much about computers. thanks guys and i was actually goin to get the XFX - ATI Radeon HD 5770 1GB GDDR5 PCI Express Graphics Card...is that the best for the $?
so i upgraded my power suply, got 2 more gigs or ram and got the HIS ati radeon 5770 and it fit perfect. any bigger and it wouldnt have. tested it out. it is 1 million times better then my mobo pos video card. its like playing completely different games now! its awesome. thanks a lot for you help guys!