Being hung up on interpolation isn't helping you any, stop thinking that interpolation = loss of quality, it's certainly not true.
I can prove it in that the following cameras give an unrezzed print size of:
20D, 11.7X7.8"
1Ds, 13.5X9"
5D, 14.5X9.7"
D2X, 14.3X9.5"
All at 300DPI. I can't give you the figures of other cameras, these are the figures taken from files I have on my computer.
Obviously to say that the 1Ds is limited to 12X8" prints is so ludicrous at to be laughable. Many say that the D2X gives the 1Ds mkII a decent run for its money despite the megapixel disadvantage, when the D2X file is rezzed up to match the larger file!
At 250DPI which is plenty for most applications the 20D/1D mkII will give you an unrezzed print of 14X9.5" which is pretty damn good for starting off with.
Checking other peoples photos at print doesn't help either. I have an incredible 18X12" print from the 10D shot at 1/10 iso 1600 that beats Velvia 100 on a 645 for cleanliness and quality if not pure resolution (I used to shoot with a Mamiya). The reason was that I was using a sharp very wide angle lens on a crop camera at its optimum aperture using MLU and a cable release, supported on a heavy tripod and heavy head with an excellent RRS L plate. The exposure was utterly perfect to a 1/10 of a stop.
My point is that unless you are comparing resolution from photos taken under the most stringent of testing conditions, and conditions that are equal to each other, you will never know what your camera is capable of. If any of the points above had been deficient then the photo would not be the best seller that it is.
For your own sanity, do yourself a favour and go to your local store with a compact flash card and your favorite lens. Shoot the 1D mkII and 5D with the same lens using your most used iso setting and usual fstop/aperture. Take a friend to model or shoot the salesman with the kind of composition and framing you usually use for bridal portraits. Take the card home and see which one has enough resolution, in print, for the kind of photos you shoot and at the enlargements you have to deal with. Then make the decision.
I did the same thing with the 5D vs the D2X and it showed me all I needed to know about the look, resolution, noise, etc of each camera, as it is relevant to my kind of shooting. That is the only relevant answer anyone can give you, not comparisons of unrezzed file sizes.
(If anyone is interested in what I found, the 5D has very slightly more resolution, slightly better noise at iso 400 and suprisingly as accurate flash, but the D2X files were far more film like, better at facial tones, more accurate at WB, far faster at low light AF with a flash than my 1Ds even and overall a better thought out camera ergonomically, but then that's a topic for another day....)