The MX 4000 is an AGP card. Runs up to 8X speed.

I'm currently trying to decide whether to upgrade to a faster card for
better 3D, or build a new system. Right now I have one of the early
Athlon 64's at about 2.2 ghz.

It does pretty much all I need (I am not a game player) so I am
reluctant to invest in another round of computer building, but at the
same time AGP is a dying format and wonder if I should just go ahead
and build a computer as opposed to sinking money into a dead end video
card.

In the old days it was easy. Dad would build a new go faster computer
and one of the kids would "inherit". Now the youngest is 21, and they
are not so easily bought off. Youngest one has a 6 month old go fast
Toshiba lap top. He is not real interested in my 4 year old computer.

Oh for the days of small children with cheap dreams...