It depends what you want to run on it. CPU-heavy games started popping up in the early 2000s, so I wouldn't recommend using that system for anything too strenuous, unless you're really not picky about frame rates.
I had an Athlon 750Mhz Slot A back then and at first used a Voodoo 3 2000 PCI (carried over from my previous system) then later upgraded to a Geforce 2 GTS 32MB. Any graphics card faster than that will allow higher resolutions in older games, but newer games (2002 and later) will likely not perform their best on a 750Mhz P3, so I wouldn't expect a 5950 Ultra or Ti 4800SE to really be able to stretch their legs much. If you have no intention of building a faster\newer AGP system any time soon, then by all means use the FX 5950 for the absolute maximum performance possible. It will be severely CPU limited but that just means you'll be able to run nearly all games at the highest resolutions and force anti aliasing without any performance loss. All of those cards will have very similar game compatibility.
For the most "period correct" card, the Geforce 2 MX would be the closest though definitely not the fastest option at the time. The MX420 will be significantly faster (ignore the specs, look at benchmarks... the 4MX series are very fast for older games because of architectural improvements) and likely be closer to the Geforce 2 GTS I had paired with my Athlon 750 back then. The 4800 and 5950 will be many times faster than any of these in later games.
Time Machine = FIC PA-2013 2.1 - K6-2 500 - 256MB PC-100 - TNT2 Pro 16MB AGP - Labway Yamaha YMF719-E - Midiman MM401
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