Go for a GTS instead of an MX (400 or 200). Even better is the PRO, which is a GTS that basically is overclocked somewhat.
The GTS has much better memory on the card and therefore will allow higher framerates and higher (with playable framerate) resolutions.
Don't spend a bunch more for 64MB of ram, no current game (or even future ones for some time) are going to need more than 32. If you find you can get a GF2 GTS card with 32 MB of ram, and an MX 400 with 64 MB of ram, pick the GTS, it will be a LOT faster.