Hi ! Yeah, I got the Canon. A 20D in the end. I found the 350D just too small for my big hands, LOL and I wanted greater 'old school' manual control IYSWIM. I picked up a fast 50mm lens to play with and have shot about 5000+ frames in the last few months (mostly of dd, LOL). The multiple shot mode is absolutely fantastic. It makes editing a 'stack' of shots tedious and hard (is this frame better than that one, which has the best focus etc. etc.) but it becomes much easier to get a keeper from a burst of 4 or 5 IYSWIM.
Photos are here if you're interested !
Out of the box, I am sure your camera is on large / high quality (maybe not 'superfine' but 'fine' nonetheless). Read the manual to check, LOL !
Photobox offers 6X4.5 printing, a size I'd never heard of, but now you mention it I guess some cameras output at this pixel ratio.
I don't know / can't quite follow the exact nature of your problem, but you shouldn't really be 'resizing' to print... more 'constraining ratios' IYSWIM... You don't want to throw any pixels full of information IYSWIM, rather just tell it how big an area you want to 'stretch' your pixel data to...
A 72ppi print will look OK on screen and crap on paper.
Also the 432 X 324 you mention is a very very tiny image (print wise).
(obviously the two are linked: 72ppi X 6 inches = 432 wide IYSWIM.
I think PE is in a mode / has a setting where it thinks you are outputting for the Web, where 72ppi would be normal... I imagine you need to get in there and set it up for high-end printing where the dpi would be 300+ IYSWIM.
You CAN'T just change ppi on an existing image that you are editing that is, say, 72dpi, (ie. upping it to 300) WITHOUT having it shrink !
You would need to reimport the origianl (LARGE) file and resize / change ppi / print from that.
Let me know if that helped any or if you solved the problem by some other means.
Check what files size you are importing. They should be, at a guess, at least 1.5MB JPEGs, quite possibly more like 2 or 3MB...