I donno, but you can actually do something about the graphics, do you still have the game frozen on 640x480 ? I don't know but for like 800Mhz pc's, 800x600 is probably depending a bit on you graphics accelerator, already the standard resolution for games. With the lod thing you can improve things a lot, also from all the blocks you use, all the sides that aren't visible; give them the 'none' flag. A lot of people tend to forget this, but it really helps. Well these are some things you can do, without raising the minimum specifications for the game..

Good luck!