if you are limited on budget, don't waste money on expensive camera and a cheap kit lens. It is better to spend money on the better lens and get a cheaper camera. I would rather have a Canon 350 or 400 with the great EF-S 17-55 F/2.8 IS USM lens, than put the money on a 40D or 50D but all I have left is a little cash for the cheapest kit lens.
I would prefer a 450D with 50/1.4 lens over a 50D with the cheap zoom lens supplied in the 450D kit
one other little detail between XT, XTi, XSi etc : some of them take the larger compact flash cards, while some other use the smaller SD cards. Now, since I have an ASUS Eee PC netbook 1000h which comes with built-in SD card reader, the choice is obvious. The 450D uses SD cards and thus I can read and transfer the images without having to play with USB cables, just take out the card and slide it into the netbook. Another advantage of portability .
I advocate prime lenses because :
-they are often cheaper
-they are often faster ( better in low light, low f/stop number)
-they are often sharper
-when you buy a zoom you will often use it mostly at one end of the zoom
-the zoom lens is often heavier, bigger, and more costly
-if you want a good zoom lens quality then it is not cheap at all.
-dust : when using a zoom microdust is pumped in and out
check out
http://www.bobatkins.com/photography/reviews/best_canon_eos_lenses.html
with a few exceptions, the best lenses are prime lenses, and some of them are very affordable : 50/1.4 , 85/1.8 , 100/2
and even in the high-end L series they can be found at a reasonable cost : 135 f/2 L , 200 f/2.8 L . ( check out Adorama or B&H used lens sales for bargains).
today, one of the best cameras from Canon are the 5D and 50D. But camera competition changes frequently. A good lens will last decades.
in the past 5 years I went from 300D to 350D and now to 450D. Three cameras in 5 years ! But my oldest lens, the 50/1.4 , is 13 years old ! Good lenses stay for decades.
Today, if I had to buy a new camera and money was no issue, I would not get the 500D, nor the 5D MkII. I would get the 50D. And the 135 f/2 L ....