Here's a comparison of a couple of dozen video cards on a P3-800. That shouldn't be too far from what you're looking for. They also tested with a K7-1200, so you can see the difference that the CPU makes.
Expect older video card prices to fall when the new GeForce4 line arrives next week. The best deal I've seen recently was for a GeForce2 GTS (not MX) for $49.
i7-4770K, H70, Gryphon Z87, 16 GiB, HD7950, SSD, 2 HD, Blu-ray, SB ZX, TJ08-E, SS-660XP², 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD