Flashing trees: Happens on all video cards. Adjustments to AA can fix this (for whatever reason, this game seems to prefer none, or x2 AA; people who have no AA have seen performances increases by turning AA on to x2) and the flickering can mitigated by lowering AA. Stupid, I agree.

Shadow issues: Not exclusive to ATI. They just have really piss-poor shadowing on characters.

Low Frame Rates In Town: 9/10 caused by your hard drive, not your video card.