Every year a new version of the digital series is launched.
The latest is 450D and the year before lasts is 350D, more mega pixels and may be a different sensor.
Most are minor upgrades, the major difference b/w a 350D Vs 400D, other than sensor & MP, is that 400D has a self sensor cleaning unit in it, faster continous mode,LCD is 2.5 inches(1.8 in 350D),9-point AF system from 30D
The major difference b/w the 350D,400D Vs the 450D, other than sensor & MP, is that 450D also has spot metering that the other 2 camera's don't have,digic III processor, 3 inch display,Larger viewfinder,Live view, 9-point AF system from 40D,ISO setting display in viewfinder,higher capacity battery, contrast detect AF and a function that boosts shadows after shooting and has a upgraded menu system, +3_-3 Ev, comparatively faster burst mode.
Looks like a bit but relatively it wouldn't make a great deal of difference, if one gets a 350D or a 400D or 450D unless one puts the cam to extreme use.