In my city world, as big as it is (34,000 quants diameter), the frame rate is extremely high, even with the whole level in view, because I mass-combined a ton of entities into one huge entity. Sprites are bad when it comes to the entity count, so avoid those and stick with models. Having one entity is about the same as having a model of about 120 polys or something. I ran an experiment a little while ago, I had 18 models on the screen at once with 4096 polys each and I was getting 18 fps. I also had a case of 624 models on the screen at once with only four polys and got basically the same frame rate. So, when making models, combine them so that you get at least 120 polys.