i would really like to know what you're expecting to run on that machine.. if its games youre talking about (hence wanting faster 3d card) which games exactly? i cant think of a single program that would have any benefit from adding a couple of extra MHz onto a TNT2. even if you threw a Radeon 9700 in there you wouldnt notice the slightest improvement. Your problem isnt the card, its the stuff the cards connected to...
EDO RAM

!!!!
thats gotta be the biggest bottleneck i ever heard of.....
so, in a nutshell..
DONT BOTHER
youre wasting perfectly good time and risking damaging a vid card that could go in a slightly better machine someday...