I don't think the T1i would do much for you, it would be very similar to the 400D.
The T2i seems to be (along with the 7D and 60D) the first big step forward in low noise among Canon's APS-C bodies in a long time (since the 10D, really). That might be worth getting.
OTOH, do you have a speedlight? If not, that might be money better spent.
Also, both of your lenses are very slow. Depending on what you shoot, either adding some fast primes at certain lengths or trading your 18-55 for a faster zoom in the same basic range might be a better idea.
The best thing to do depends on what you are shooting and why you are wanting an upgrade in the first place. What specifically are you hoping to get from the new body?