Epaminondas,
I don't know what turn processing time is unbearable for you, but it should be in the 30 sec-1 min range. With a 1.5GHz processor you shouldn't expect super performances..
About battle screen I had also very slow framerates with my old Radeon9600 card (on a AMD XP2400) when graphics options where set to medium+ (the framerate was correct - 20 or so fps- on smaller battles). Now with a GF6600 I don't have any slowdown anymore even when spells fly by scores.
So set the graphics options at the minimum and you should have acceptable speed (and frankly it's not *that* worse graphics-wise)
