I don't think your machine will run it well, because the Direct3d mode is terrible and the OpenGL mode is unsupported and very unstable. My friends p350 64mb voodoo3 ran it better than my tbird1Ghz 384mb TNT2ultra. hopefully there will be a new patch soon though.
due to the fact that the first generation unreal engine is being used in deus ex, i think you will have no trouble at lower resolution, and try making your way up then.
i have a PIII 550, and a geforce 256, with 256megs of ram , and the game is reasonably good.
What always boggled my mind was the fact that the game just didn't seem very fluid when I played it. I have a PIII 866 with 256mb Crucial RAM, along with the 64mb GTS card, and yet even at 1024x768 in 32 bit color, it just wasn't fluid...erked me a lot. At the same settings for Q3 or UT, movement is butter smooth. The game and plot is certainly awesome though =) I love divergent endings.
make sure all your video card drivers are updated. I recently had problems with my GEFORCE 2 GTS and Deus Ex (BTW, Deus Ex is using a modified version of the Unreal Tournament engine I do believe) and I updated to the detonator3 12.41 drivers. Took any and all slowness away from Deus Ex...even though I was in direct3d. Blech.
Ran like clockwork on my other computer with the Voodoo5 5500.
I've been using the 12.41 drivers. I try to keep up with the tweaking community. I still haven't managed to isolate the reason for the non-fluidity. Compared to QIII it's very distinct, but after playing for a while, it fades into the background of annoyance.
I believe you're right on the UT engine usage though, the original Unreal engine doesn't look nearly that good.