Initial question is: *which* FX 5700? There are many variants. That said, perhaps this will give you some insight: AGP Mini-Comparison (7 Cards, 3DMark2001) **UPDATED** (I tested a 5700U).
The P4 I used is about on-par with very fast Pentium 3/Athlon chips; your TOTL P3 may even be a little bit faster. For comparison, here's a review with a 5700U paired with a faster system: http://www.bjorn3d.com/2003/10/bfg-geforce-fx … -fx-5950-ultra/ (an AthlonXP 2500+ with DDR and an nForce 2).
Unfortunately they only show raw score in 3D01, but it is somewhat higher (my guess is, however, that the more GPU-bound tests aren't moving very much, and the score gain is from CPU-bound tests being able to run even faster). So basically to your question: will it bottleneck? sure. Will it matter? probably not. If you were trying to run DX9 games that are at the limit for the 5700 (Oblivion, for example, lists the FX 5700 as its minimum system requirements) the P3 would probably be an issue, but for older games (let's use Morrowind as an example) it will still be an improvement over the V3 (especially since it will add PS support). KOTOR2 is a big question mark though - I'm unfamiliar with the game, but it's fairly new for this kind of machine (it came out in 2005), and lists a 1GHz P3 as its minimum system requirements; it may not run very well, but it should run (as you do technically meet all of the requirements with the 5700 in place).
I think the bigger hang-up is if you're going from a Voodoo3 3000 to a Voodoo Graphics 4MB for Glide; that's a pretty big step down depending on what you're trying to run. A Voodoo2 or pair of Voodoo2s would be a better replacement candidate imho.