I have just 2 questions. 1) Define a 'normal game'. 2) What CPU have you got?
In terms of graphics power, the GF2 MX has the TNT2 beat however, you will not see an amazing difference in speed going to an MX from a TNT2 Ultra. Your frames per second will go up in 3D games a bit, but it wouldn't be a great increase. Dont be fooled by the 3DMark2000/1 scores, that will just utilise the cards T&L and boost the score, which is not truely representitive of most games.
If you mean a normal game as in old DOS 2D games, then these are really limited by processing power. A graphics card change may help a little, but everything that could be done for speed in 2D games was pretty much already done on the TNT series.
A suitable cheap upgrade from a TNT2 Ultra would be more along the lines of a GF2 if you really want to see any real difference.