Hi Trevor
The 350D is a great camera for entry level DSLR. The main problem is the kit lens it is normally shipped with is complete pants, so if purchasing one, you would very soon want to upgrade. It also doesn't come with the "sensor cleaning", which I believe both of the later models do. My 50D has this feature and it really does cut down the number of times you need to blow dust off the sensor.
Something else to consider, the 450 has live view, the 400 & 350 don't (something I am now using extensively for landscape work, that initially I was very sceptical of). The 450 has a 3.0" LCD screen rather than the 2.5" of the 400. The 450 has the Digic III processor, rather than the Digic II of the 400 & 350. I also understand the 450 has a better kit lens as standard, though that would be dependant on which deal you went for.
If it was my money, I am pretty sure the 450 would be the one I would choose, though I started with the 350 over the 400 just to "get into it" for less money, and then progressed from there.