You probably won't be able to find anything brand new that will work with your Pentium 3 system. If you can find one used and you have an AGP slot in your machine, an ATI Radeon 9800 pro or an NVidia GeForce TI 4600 would be about the best cards you could put in your system. Even if you find something to upgrade your old Rage Pro card, Company of Heroes and Gothic 3 will require much more powerful processor than your Pentium 3.
As it sounds like you don't want to invest much money in to a new computer, my advice is this. People practically just throw away old Pentium 4 or Athlon XP systems these days. You could probably pick one up for super cheap (or possibly even free) on Craigslist or the classified ads in your newspaper. If you can find an Athlon 64 or Core 2 Duo system, that would be even better. Then just pick up a used PCIe graphics card with at least 512mb of RAM.