Keep the 18-55mm lense, IMO. That lense is not too bad at all - I own a D-rebel and the lense is essentially a life-saver. I own the 50mm f/1.8 too, but because it's a prime lense (fixed focal length?) you wouldn't be using it too often unless for night photography (applies to a wedding?), low light condition (large maximum f/) or unless you are those who are keen on shooting with perspective

. Seriously, at 70mm on the XT there's not much you can do with it, because it's no longer a "normal" lense...
Until you can afford yourself something better, you might as well stay put with the lense. . In fact, because 18mm is a wide-angle f.l ... on your D-rebel XT its about 29mm on your camera, minimum. With the magnification factor on the XT, that's probably the best focal length for a budget price (since you're buying the body with the lense). If you opt otherwise, you'll be looking at the 10-22mm USM... which is really pricey. Use the
bhphotovideo.com link below to check it out.
Of course, if you can afford an "L" lense, forget everything I've just said about the lense.

Also, if you're doing photography for a living you better not comprimise by using the lense
EDIT - just checked out the Sigma 18-125mm lense. Great lense if you can afford it.I'm only posting on the assumption that you're only shooting for casual.