When you go into advanced display settings does it show options for rotating the display or do you have a pivot program showing in the advanced options tabs?
If so, that's what will do it for you. I'm running a Lenovo Thinkcentre (Intel graphics chipset) with a Gateway monitor (1440 x 900). And boy does it look weird running sideways.
I should think the ATI would have enough oomph to do it.
I assume you've checked the various tech boards. There might be a driver issue - see if there are newer drivers for your graphics card.
