I find myself in the exact same situation. I am also running an add-on Diamond M3d II (8Mb). From the research I've done, the choices aren't easy.
The Voodoo3 set looks very impresive. Early benchmarks show that it will at least give the TNT 2 a decent competition. The fact that the V3 architecture doesn't support 32 bit color depth shouldn't be too discouraging so long as 3Dfx manages to maintain a good picture quality (which it has done with every card except the Banshee). Interestingly enough, it appears that the latest drivers for the V3 set have improved performance quite a bit (this goes to show that the prvious drivers for these cards have not taken full advantage of the V3 hardware, and that any performance benchmarks already published should be, at best, viewed as baselines which will slowly improve in the future.)
The TNT2 looks good as well. While it hasn't fully matured yet, the wait may well be worth it. A look at current benchmarks show astounding performance to say the least. Other than that, substantial, stable information isn't really available for this product yet. Give it a few weeks. (Riva will probably want to hit the market as close to the release of the V3 3500 as possible)
Perhaps the major factor to consider here
is the environment into which these cards will be introduced. TNT based video cards have shown themselves to be much more CPU dependant than their Voodoo counterparts (not a good thing for early Pentium and AMD users). On the plus side, this translates into a large performance boost if you plan to slap this card into a power system. Voodoo based cards are less biased than this. You won't see the same performance jump from a state-of-the-art system to a I-could-of-bought-a-speak-N-spell system. Voodoo based cards also have a history of running hot. Not a big deal, but this should be a consideration none-the-less.
If you look hard enough, you should be able to find a site that sells a M3d II (8Mb)
Or Try this: