Depends on your budget to be honest. Personally speaking (and also a few friends) the 28-90 you have is not a good partner.
If you are low on funds look up on ebay for the original kit lens, maybe even the updated version, so look for the 18-55 Canon lens. The updated II version is meant to be very good - and it will cost nothing (as lenses go).
People overlook the kit lens too much in my opinion.
However, in this case, you'll find it'll be much easier to use as it is __so__ much wider.
If you do have money to spare, then look up something like the Tokina 11-16mm to scratch the ultra wide angle itch. Got one a year ago and it's my "weapon of choice" for nearly everything I do.
Another thing with the original Rebel, is shoot in RAW if you can. It'll take a nice big CF card so get one off Ebay. It'll only count to 999 images, but don't worry about that.
With RAW you will eat the memory card
However, the reason I mention RAW is that the original Rebel (300D) in jpeg can be a bit hit and miss with exposure. If you shoot in RAW on that body you should get a better dynamic range.
It's worth comparing the two images, RAW vs JPEG L.
If you are downloading to a Mac (eg iPhoto), iPhoto will deal with all the technical mumbo jumbo of RAW.