At the moment, the graphics is a huge bottleneck - if I read it correctly, the card is a
Geforce2 Mx400 - a budget version of Nvidia's Geforce 2, and a card dating back to the DirectX 7 era - it DOES have hardware T&L, but not much else.
A Pentium 1.6 GHz is not exactly top of the line, but only around half of today's, rather than some ancient old clunker.
256Mb of RAM may cause some pain, and while with dual channel RDRAM (if rigged for dual channel), it will be fast RAM, buying a memory upgrade would be even more pain.
One of today's "budget" cards would seriously outclass the old 2MX, and breathe fire into anything that runs already, but going above a low to midrange card would end up unbalanced the other way .. and forget todays graphic-monster games.
The Geforce 4 Ti was a force in its day, full DX8 hardware, at a time when Nvidia had ATI on the ropes, and is worth a look if found cheap (forget the Geforce 4 MX, it's closer to a full Geforce 2 than anything else - a lot quicker than the 2 MX400, but no more capable).
Just depends, best value would be to hold back until you can go for a balanced full system upgrade, otherwise give us a few price examples and we'll pick over the bones.