The reason is that you're using a Direct X 8.0 card... Direct X 8.1 (the real DX8 cards) support hardware T&L and better pixel shaders. The same is with the Geforce 4 MX cards, even though they are a "4" series, they still are slower than the Geforce 3 Ti cards, and even the Geforce 2 GTS !!
Doom 3 is a Direct X 9 game, so you at least need a "real" DX8 card just to be able to play it...
If I were you, I'd look into buying a card in the Geforce 6800 series or better, or Radeon x800/x1800 series or better - IF I were on a seriously tight budget and didn't play many games.
IF you have a fairly fast CPU (by fast I mean single core Athlon 64, Pentium 4 3.2GHz or better), you could upgrade to a fast AGP graphics card (you obviously can't get a PCI-Express card unless you upgrade your motherboard) like the Radeon HD 3850. Then you could play any game on the market right now, including Crysis, but as mentioned, the rest of your system must be up to the task.
Bottom line, throw that Geforce 2 MX in the trash, it's worthless. I have an old Geforce 4 Ti4600 128MB DDR AGP8X card and an old Radeon 9000 Pro 128MB card I can sell for cheap that'll run that game. E-mail me if you need to.