The irony is that the Atlanta's a shithole. Good food (but where is that difficult to find in Thailand?), a nice pool (again not difficult to find), and a grand entrance hall, but crumbling, shabby rooms. It's also not in a particularly nice soi. All those signs all over the place telling you how lucky
you are to be staying there are a bit too much.
Suk 11 is the best budget place in Sukhumvit by quite a long way. It's not *really* backpacker/budget territory, so it's either going to be either by the hour or luxury so your options are somewhat limited. (Although moving up towards Siam, with the aforementioned Wendy House, Reno ... and the others down that Soi might be worth looking at.) Thonglor is probably worth checking out as well for more intermediate lodgings if you're set against Khao San.
Travelfish is one of the better review sites.