Those specs seem enough to run GLXtreem quite well (I run it on a P2 600Mhtz, 128MB ram and a 64MB Geforce 4) without a problem.

One problem on GLXtreem seems to be color depth - I havn;t quite worked out what the problem is but check that your screen settings are on 24bit color as this seems to make some difference.

Out of interest - try download Farmer Joe's matching game from my web site and see if it also runs very slowly.