For N64 emulation, good antialiasing and ansiotropic filtering make a big difference. The M9 (Radeon 9000) is breathtaking in comparison to my old M4. With ansiotropic filtering at 16x, things get a little blurry, so for some 8x might be preferable (I use 16x, though.) With that and the antialiasing, it makes the N64 look very clean and higher detailed than it is, I really like it.
As I said, i'm nearly sure the Radeon 9000 (non-pro) is cheaper than the ti 4200, and it is sufficiently fast and has all the features that the MX 4 does not, including full DX 8.1 support (which was removed from the MX 4,) with pixel shader support. It's far improved from the MX 4 and only a little more expensive (not 100% sure on the price.)