Have the same issue with my nforce2 system as you mentioned above. I am stuck at roughly the 400fps mark. Can't be right at all for glxgears, since I get some 70-75 fps in games using Windows 2000 Pro. All libraries and modules are correct, and it's the latest drivers.
Specs are Athlon XP1600+ (at 1.9GHz), 512Mb RAM 3-2-2 CL2 at 190FSB, MSI K7N2G-L mobo with internal GF3.5 (it's not really a GF4) mx set to utilize 128Mb of RAM.
It outta be faster than that even though it is an internal GPU using the RAM at 'only' 190FSB
(Oh, and my 1280x960-mode disappeared when I updated the drivers to the latest version. It doesn't matter if I do the "NoBandWidthTest" or not. And, where is that nforce2-update? IDE-performance is baaad, man, really bad. Way baad. I think I'll see if other ppl have this too. *off to search the forum*)