I have a new 14" and have the hack installed. It works with my 17" 1280x1024 LCD. The only bug I found was my clock resetting on each reboot. I zapped the pram and retried the hack and I no longer have the problem. I think the clock reset may have to do with having the video adapter plugged into the iBook without the second monitor connected.
I havent had any major slowdown effects except for when I am dragging from screen to screen. The edges of the application window have a jagged effect, but if I am just moving it within my secondary monitor, it is as smooth as on my iBook. Importantly, Expose and Dashboard run smooth. You dont have anything to lose. All those spazzes who scare people off boards are usually misinformed with older iBook (Rage card) stories or freak incidents.