I've got the AD11 motherboard in my computer too, and I have had several different video cards in it to test out. The Radeon line of video cards have a lot of power and certainly great potential, but as history keeps repeating itself, ATI does not have a clue how to write drivers for their hardware. I'd love to buy one of their cards, especially their all-in-wonder card, but without good drivers the hardware is more trouble than it's worth.
For the differences between the Geforce cards, the MX line of chips are marketted more for business users, as a lot of the 3D optimizations of the Geforce chip has been disabled. This doesn't mean it's bad, as it still performes better than the original Geforce chips. In my opinion, if you think you just need one of the MX chips, go with the MX200 as that extra 32meg of memory on the MX400 is wasted away unless you play a 3D game at 1024x768x32 colors - which if you were you'd buy the GTS or Ti chip instead.
The GTS is nVidia's performance chip which offers excellent performance, then the Ti was released to combat ATI's new line of Radeon cards, given the Geforce cards a little extra power.
No matter which card you buy, the MX, GTS, or Ti, they all perform very well, it's just a question of what your use is. If you use the computer mainly for business applications or surfing the web, and even the occasional 3D game, buy a card based off the MX200 line. If you want to play Quake 3 online with 15 other players and a resolution of 1280x1024 and want to smoke them all away, then look towards the GTS or the Ti, and just decide how much money you're willing to spend.
Myselft, I've got a Geforce2 GTS Pro with 32meg ram.