maybe i have a solution for your problem.
I also have a notebook with ATI Mobility X1300 and I installed Windows 7-64 in the last days. And first there was no posibility to change to 1280x800.
But I found a tool to solve the problem.
I hope it is ok in this forum to post the link, it is ATI MobilityModder - DriverHeaven.net
And i used "ATI Catalyst 7.11 Vista (64-bit)" from guru3d.com