Why do you need to upgrade? What is it that the T1i won't do? Without the answers to these questions no one can really advise you.
A lot of people are on here asking about upgrading from a Xti or Xsi, in which case they can get much better high-ISO performance from the newer Rebels, but the T1i is sort of intermediate, better than the earlier models, not as good as the ones after it (starting with T2i). If high-ISO performance is not of great importance to you, then you won't gain any big advantage on that front.
There's also an increase in megapixel count in later models, but the T1i should have enough for almost anyone.