I have a G4-733, so it's got that Geforce2.
A Geforce 2 should be able to run Diablo 2 just fine. Maybe it has to do with something else in your system? How much RAM do you have?
How does Diablo 2 not run decently? Do you consistently get low frame rates (is it jerky all the time) or do you get more or less smooth frames but with a slight pause every now and then. Or does it just frame up with lots of action or on certain maps?
I know with my old Celeron 533 machine with a 32Mb TNT2, Diablo 2 framed up badly with lots of action and even with no action, there was an annoying pause or jerkiness every now and then. I attributed this to:
1. Not enough RAM (had 128Mb - 384 Mb helped somewhat) & hitting VM on occasion;
2. Slow CPU with not enough on-chip cache (I think the CPU speed was probably OK, but the small cache could have caused some slow-down);
3. Slow video card - but considering Diablo 2 was 2D only (except for some 3D lighting effects & perspective drawing if enabled) it should have been fine; 2D speed hasn't increased all that much from the TNT2 to Geforce2/3/4 (most of the R&D goes into 3D since that is where everything is going);
4. Combination of all of the above.
A 333Mhz Celeron compaq laptop I once knew had integrated graphics (obviously) and only 128Mb of RAM, and was virtually unplayable with even some action on-screen. There would be huge long pauses in a multiplayer game just when monsters come into view. By the next frame update, the player was already dead.
[Edit: actually, I think the laptop only had 64Mb - which was probably 90% of the reason for why it was unbearable.]
Diablo 2 performance was much better on my old P3 866 Mhz with the same RAM and TNT2 but there were still pauses every now and then. It is silky smooth all the way on my 1.67 GHz Athlon with 1Gb RAM and Ti4200 (not bragging, just a comparison).
Is your 733 G4 the model with the missing L2 cache (or is it the L3 - can't remember)? I don't think this should make much of a difference though. Most probably (and without knowing how much RAM you have or what extensions are loaded or background processes etc you are running), I'd say increasing the RAM should help *a lot* (stop hitting the virtual memory - multiplayer games take up lots more memory than single player)). If you have 512 MB or more, then just ignore what I just said (but it may be relevant for multiplayer games with lots of players).