I'm not 100% up on ATI stuff, but usually "SE" for an ATI card means that it is a budget version, usually with fewer pipelines, slower clock, less memory and/or memory bandwidth than a non-SE version. The X800SE looks like it will only be available as OEM to Dell, Gateway, etc and not directly to retail customers.
I'm guessing that the X800SE only has 8 pipelines and 128M, compared with the 12 pipelines and 256M on the retail X800 Pro, and possibly a slower clock and smaller memory bus. It should work pretty well for you, but not as fast as a full X800 Pro and with less memory. It may be close to a 9800 Pro speed-wise.
Hey just found this chart:http://www.hardocp.com/image.html?image=MTA4MTgxOTA5NXhEeE5HbnF3RzFfMV8xX2wuZ2lm