I really dig on MS Combat Flight Sim, both online and off. I also run a K6-2 350. I installed a CL TNT card recently and it works great. Nice and smooth at 800x600x16 with all game settings set to the maximum (realism, texturing, etc.)
Most new flight sims are using Direct3D API. so there aren't any need to get a 3Dfx card. Any decent 3D card will be fine, such as I740 and TNT. I have V2 myself and is happy with it too. I think your system is fine for most sims except if you want to try out games like Jane's IAF and WWII Fighters.
I think would be better if you buy a video card that has drivers to support your 3DNow! CPU, that way 3D games will benefit from it.