One thing to watch when buying heavy machines off ebay are delivery cost and insurance.
If the emac is between £200-300, I’d buy a Mac Mini instead - they are about that price on ebay and in fact just £330 from Apple with 512MB Ram. It has a 4x 32MB card too. I found a 700MHz emac on ebay for $395 (£228) with just 128MB Ram and a CD Rom. That’s really not a good deal considering postage is $45.
The card memory doesn’t really have anything to do with the number of verts - that depends on your card model. The VRam is used for storing textures. I can’t remember but I think some drawing options like vertex arrays cache the verts in VRam. That really only speeds up games though because the models don’t change. For Blender, it makes little difference. There is actually a vertex array option in Blender but it doesn’t affect performance much at all.
My Mini with Radeon 9200 can handle models with about 100,000 verts each but it can support scenes with a few of these objects, so overall, I can get it up to over 1 million verts in a scene - though it’s sluggish. If you use subdivs, then you can obviously get much more and the limitation there is your RAM. 1 GB Ram is limited to about 2 million verts anyway.