Well, at least this system has a lot of ram. (I hope it's DDR ram though.

)
The gfx card ist not that bad. If it has 64 MB ram, texture sizes (as long as you use compressed maps) should not affect the performance.
Ok, here are my tricks to improve the overall performance:
-Use only compressed maps. (S3TC for the main map, DXT5 for the shinemap.)
-If your model has more than one map, try to reduce the number of total maps for lower lods.
-Try to avoid the ani format. It's bad for systems with a slow CPU.
Rather use EFFs (S3TC) for effects and glowmaps.
-Try not to use too many lods. Systems with with a small ammount of ram will rather run out of memory than gain performance.
-Use PCX for effects that only need around 256 colors.
-Some effects can look ok with S3TC compression too.
-Use compressed maps for skyboxes. If the compression looks horrible, double the size of the map. It will still be smaller.
(Double size = size*4 / S3TC = size/8)
-Use S3TC for big glowmaps rather than PCX.