I haven't been mapping for Tremulous much, unfortunately (Whatever I do I cannot get the right entities to pop up, even to the point of editing every entities.def I can find)...I don't think my computer can really handel tremulous anyways
(You try playing it on a pentium III with a 32 meg video card...ever hit 2 fps?)
It's based on Quake3. Should run fine on an 8MB Voodoo3 and a PII400 w/64MB of RAM (probably want significantly more if you are running an OS which is heavier than Win98 or stripped down Linux), turn the quality down--eg, run at 640x480 and tune the GL settings to your video card. For instance, start at the 'low quality' setting and then adjust certain things if you know they are wrong (for instance, nVidia cards should always run in 32bit color and never 16 bit, while Voodoo cards should run 16bit and never 32bit). If your frame rate jumps, try boosting your quality until you get the ballance of visuals vs. performance that you like.
I ran Q3 with a Celery366, 96MB RAM, 8MB Voodoo3 2000 running a performance tuned install of Slackware Linux and got solid, playable frame rates. I can't give you numbers, but I never noticed any problems with smoothness, I recently upgraded my machine, since I'm no longer a poor college student.
However, on my PIII750 Laptop w/8MB Mach64 video card, I do hit 2FPS or worse on the intro animation, but that's because the screen is a single resolution LCD (1024x768) and the video chipset is from the generation before ATI had a Q3 capable card (Rage128). I get 9FPS in game, but I'm just running around testing, so it isn't a big deal. I haven't messed around with running it windowed yet. However, any PIII is well above what counted as high-end when the engine was originally released in 1999.
Jeff