If you want to be able to play today and tomorrow's games you will need to get a new video card. The one that would guarantee good performance in today and tomorrow's games would be the Riva TNT. The TNT offers much higher resolutions, better image quality, AGP 2X, and 16mb of ram. Voodoo offers framerates in the same area as TNT, but with a max resolution of 800x600, 8mb or 12mb versions, and Glide support (something TNT doesn't support), and is only avaible in PCI.
If you really want Glide support, a very viable alternative to a voodoo2 is a voodoo1. They can be had for as little as $40 in some places and offer excellent performance in Glide applications (mostly DOS games), and it will work flawlessly with a almost any video card.
If you already have a good 2D card I'd go with a Voodoo2. If, however, you are trying to conserve PCI slots and would like to keep the extra openning go for the TNT 2D/3D card. I personally have a 2D/3D TNT card, but my system is weeks new and I needed all the PCI slots I could get. If you are worried about the difference between the quality you'll get between the two options, don't. The TNT edges out the Voodoo2 a litte, but I don't know if you'll notice all that much. Plus I'm guessing you'll probably get better 2D with a dedicated card and a Voodoo2. What most of the opinions around here and the reviews you'll find around the net focus on is the 3D benchmarks. The 2D is good on most of these cards but it would be a leap to say it is spectacular. If you want a real in depth overview on video cards go to www.tomshardware.com. If you haven't seen that advice posted here already you will soon. It seems to be the clear winner in video card info. Good luck!
If benchmarks mean anything to you I should point out that a Riva TNT card only begins to perform near voodoo2 on a P2-300 or better. I think your best option, if you're keeping your present system, is to get a MaxiGamer Phoenix (Banshee chipset). It offers the most options of both the Voodoo2 and the TNT. It doesn't support colour depth higher than 16bit or multi-texturing, but it does offer resolutions higher than 800x600 and is very fast on older systems. Don't waste time on an original voodoo card when the Banshee outperforms it for very little extra cost.