[quote="Taiyo.uk"
Slightly off-topic, but should new models have a (significantly) higher poly count?[/quote]
In fact, NO. High poly count is a BAD VERY VERY BAD thing, and compared to OA models I can assure oyu that nexuiz models have much more high polycount. High polycount puts the GPU under very big pressure, that's why your FPS significally drops when more and more players are seen at once on your screen.
The trick is, that you make a high poly version first, for renders and stuff, and then make it low poly, and use special skin effects like bumpmaps, phong shading, and much more I don't know about, but the trick is in the textures and in the shaders.
So nexuiz models are too high poly, that's why it doesn't run well on older pcs.