If your budget is maxed at $80, I would go for the Geforce 2 GTS-V from visiontek via newegg. $69 delivered and it seems all of them will overclock to match a regular GTS. Even at default it will whomp any MX regardless of 64 or 32 MB of ram, the GTS-V and its 4 pipelines vs. the 2 for the MX combined with its 128 bit memory bus and DDR memory will dominate at higher resolutions.
If you could squeeze just a bit more, I would go for a Geforce 2 Ti for around $95 for the 32 MB, or $106 for the 64 MB MSI card. Higher clocked core and memory, newer core design, worth the extra money over the GTS-V if you have the extra money! I have upgraded 2 different machines from the original MX to the GTS cards and it was worth it. I upgraded one of the 32 MB GTS machines to a 64 MB Geforce Pro Gainward 450 with the 4 ns ram, and it wasn't worth it IMHO. Should have saved the $50 and gone with the GTS-V in retrospect. Still hoping the Geforce 3 Ti 200's will drop to around $125 so they will meet my self imposed video card ceiling for the next round of upgrades.
Chuck