I have a similar setup although I have an AMD Athlon instead of a P3 and a GeForce2 GTS (still 32mb).
Do you have "backbuffer in system memory" checked? What res are you running at?
I am running at 1024x768 res (20 in monitor so it looks alright) and screen size 9, in 3D accelerated mode (and backbuffer in system memory is UNchecked). I am also running with the 1.01 JK patch and on a full install.
Do you have the latest Detonator Drivers installed and the latest DirectX7?
As far as running "too fast" in 3d mode, I don't know if you CAN go too fast with a 3d game. I get 100 fps (at the command prompt type "Framerate" with no quotes. Get to the command prompt by hitting T then tab).
Don't run in 2d mode, it will always be slower and/or look worse than 3d mode with your current setup. Smoothness may take some getting used to, but its better than slowness in the end. I know it took me awhile to get used to games like Q3A and UT being "fast" on my new system as opposed to my old one, but it lets me aim much better, and I like it.
It may just be that you aren't used to the speed. In first person view I admit, the hands move very fast, so it's much easier to use third person for saber fights. I guess you could run a bunch of other programs in the background to slow down your computer, but I figure its better to just go with the flow and get used to the new technology.
[This message has been edited by Kurgan (edited October 18, 2000).]