Well this is how I set mine up with my Radeon 9700 and HS10.
Right click on the PowerStrip icon in the taskbar thingy and select
Display Profiles followed by Configure.
Then click on Advanced Timing Options and then click on Custom Resolutions.
Then copy the below onto the Windows clipboard, highlight it then right click and select copy.
1366x768=1366,65,142,207,768,1,3,21,79046,39
Then click on the Paste timings button on the PowerStip dialog box, it's on the right hand side of the dialog box underneath the 16:10 aspect ratio.
Then click on Add New Resolution.
You will then have to reboot your machine after which you should be able to go into the display proporties and select the newly created resolution.
Hope that helps
Cheers, Lee