I have a hacked Rebel, and, while I am happy to have some of the added features, I would point out that they are in fact hacks, and only really useful because the camera lacked them in the basic mode...so if you plan to use things like mirror lock-up and whatnot that the hack enables, you are much better off with the XT, where they are native features.
I would much prefer to push the shutter release twice to lock-up and fire the shutter, as there are a lot of things that can go wrong with setting your shutter lockup on a 3s delay (as in, that could be the exact moment the wind picks up...)
Look at the features list. If you don't need the new features, don't care about the extra MP, and want to save $200, then the 300D is fine, but keep in mind there are several hardware advantages to the 350D, including instant power-on, faster write speed, faster shooting, more shots per burst, etc. that you might find extremely useful.