
As pretty much summed up, it depends upon what generation you're talking.
GF4TI are enahnced GF3 cards and superior in every way.
GF4MX are enhanced GF2 cards, still inferior in most ways (certainly speed) to GF3 cards.
GF3TI cards came in both TI200 (slower) and TI500 (slightly faster) forms but were essentially identical to the standard GF3.
GF3MX never surfaced, many thought this would have been a more apt but still shameful name for the GF4MX cards.
GF2TI are slightly faster than GF2GTS but slightly slower than GF2Ultra, but are significantly cheaper than GF2Ultra.
GF2MX have half the RAM throughput (SDR) of the equivilent GF2GTS/Pro/TI/Ultra (All DDR) which knocks perf at least 50%.

HTH!
EDIT: See how these cards line up;
TomsHW VGA Chart (Many cards)