XT to XSi is a good sized jump and not a lot of money. The XTi should sell for $200-250 (assuming good condition, you have the box, etc. ) or more if you are lucky. The XSi used is about $400 or less. XSi gives you a little bit bigger viewfinder, live view, more megapixels and a better control/menu interface. Also, it gives you auto-ISO.
That being said, jumping to the T1i adds a much better LCD and movie mode if you want it.
T2i is a jump in high ISO performance, and a number of other improvements.
d5000 is different -- it's a different feel, flip LCD better high ISO than the XT/XTi/XSi (and maybe T1i) but not the T2i.
Tough call. I'd start with price point, but the upgrade's worth it, but I'd skip the XTi. At least the XSi has auto-iso which is handy.
6D: 50, 85, 28-75, 70-210USM, 430EXii.
EOS-M: 22, 18-55