I'm soon planning on getting a Socket939 A64 3200, motherboard, and 6600GT PCI-Express video card, ripping the rest of the necessary parts from my existing machine, and it will come to a bit under $500, maybe somewhat more if I get a better, quieter power supply and a super silent CPU fan or something (noise in my current machine is driving me crazy, I will be going for silence this time around). Should be a very nice little upgrade from the underclocked (really horrendously old motherboard) AthlonXP 2500 and ATI 9600 I am rocking right now.
ATI rocks, no question about it, I've used them for my last 4 CPUs or so and have absolutely maximized the performance per dollar spent as a result.