There is a big leap from GeForce4 to GeForce FX.
If you want to see some information on the GeForce TI cards you can click
Here.
As an avid computer gamer myself.. I find the GeForce4 TI series of cards to be awesome for every game I have played using my TI 4200. No complaints whatsoever.
Here are some stats on the FX card:
500Mhz Core Clock
DDR2 1.0Ghz Memory - Fastest on the market currently for graphics GPU cards
400Mhz RAMDAC's
DVI-I
and TV OUT
And it offers TRUE 128-bit studio quality color.
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I wouldnt get an FX right now.. only because its going to be hard to find any computer games that can even use the full potential of the card "yet".
Seems hardware always moves faster than software.. and right now the GeForce4 series is still doing really well with all of the games on the market.
If you have the money.. go for an FX.. but if you just want to get a card.. a TI 4200 or 4600 will be more than sufficient.