Corrupted graphics at end of turn

Before assumptions are made, I have had this happen with vanilla as well as a couple of mods. I will be playing a campaign and after I control a certain amount of regions, the end of turn icons that slide down the left side of the screen start getting corrupted. I have had them bad enough to freeze the game because the mousovers get corrupted and unusable. I have a AMD dual core 2.4g cpu, nVidia 7500LE set on high performance, M2TW video settings set to minimum. I have been able to get a little bit more out of it by overclocking but that scares me. Heat where I live is an enemy.

I know I'm going to have to spend some money on a graphics card but the question I would like answered, even on 1024x768 settings, why are th end of turn icons the first thing to go? I only play campaign map. I don't fight battles.