Copypastan, the banner is 640x100 only if I keep the things (links and stuff) on the right.
That's for a general banner. Now if you want to remove the whole top of the page, including the "Welcome back, me!", news, message links, recent posts, and replace it with a single banner, I'm not sure what would fit. Probably something like 800x115, but I'm not totally sure.
I'd rather keep a general banner (640x100 or less), but after this it's all a matter of opinion; I don't know who uses the links on the right.
I'll put digi162's banner for now. Thanks.
EDIT: Done. Reload without cache (Ctrl+F5 in Firefox). Added a border because the picture doesn't blend with the background, but I can kick it out if you want.