new lens vs new camera

What do you photograph, and under what sort of lighting conditions?

used 40D with a Tamron 17-50 2.8 would fit in your budget but I would suggest the new t2i with the new kit lens.

"I currently have the Rebel xt(out dated) and a canon 28-90mm ef 4-5.6 and a canon 90-300mm 4.6-5.6. " ======================================================================

The Tamron 17-50 2.8 is a good sharp lens and puts you into semi/pro territory. These days I don't consider anthing over F4. This is a habit I picked up back from the film days when I had to stop shooting at dusk or use flash, because the lens could not handle it.

An XT is "old" in digital age, but still quite capable. More pixels is usually better, but it's not the pixels nor the camera OR lens that makes the picture. I have seen absolutely stunning landscapes done on an XT with the old 18-55mm kit lens.
Your lenses (if they are the ones I'm thinking of) are also older and not the top of the line, but like your camera, they are plenty capable. At present, you have no real wide angle, if these are the only lenses. The lenses and the camera body are probably worth less on eBay than they are in use to you. It's really nice to have two camera bodies, so you don't have to mess with lens changes and such in the field so much.

A used 40D is indeed perhaps the very best buy in used Canon cameras right now. Whatever, it is probably time to upgrade something. If I were you, I might think very seriously about getting the IS version of the 18-55mm lens. It looks cheap, is cheap, and does very well. Not more than $200 for the lens, and depending on what you were planing to spend, you could pick up one of the less expensive, but newer, APS-C cameras. There are very attractive bundles of the 18-55 IS with the various Rebel cameras. Check at Adorama or B&H--anybody much cheaper is suspect. Both also sell used older cameras as well.

For low light, there are some relatively inexpensive "plastic" prime lenses that can be optically very good indeed without costing much.

I mostly shoot outdoor, nature pictures.

For well under $1000 then you could get three fast primes and make a world of difference that a new camera body will not. I suggest looking at 28mm 50mm and 100mm

How big are you planning on printing?

I vote for lenses over body. The 50D has already sort of been replaced by the 7D, so in a year or two you'll be looking to upgrade your body again anyways to the latest and greatest. But lenses from 10 years ago that were of pro build and quality are still viable lenses. And I'd take wide aperture lenses over smaller aperture lens with IS any day, however IS lenses do have their place.

Maybe a Tamron 17-50 f/2.8 for starters. You didn't list anything that wide and it's always nice to add to your focal lengths when building onto your starter kit. You'd be amazed at the low light capabilities of a fast lens.

absolutely recommend a lens. try a (canon) prime on your xt. you'll be surprised what that combo can produce.
if you really want a zoom that's fine, but you can get 2 or 3 good primes for the same money. try a 28 or 35, a 50 f 1.8, and a 85 or 100.
as for the xt being 'out of date' check out juza's landscapes. when he was starting out he used a 350d and other similar rebel-type bodies with kit or consumer zooms:
http://www.juzaphoto.com/eng/juza_portfolio.htm#001881