Maximum print size for any given megapixel count depends on your level of technical nitpickyness, technique, lens, subject matter and viewing distance. Obviously, the farther the viewer is from the print, the less detailed it has to be to still look "acceptably sharp", whatever that is.
Subject matter makes a big difference as well. Simple geometric shapes shouded in fog require far fewer megapixels to render well than a landscape scene with many trees, flowers, branches, and leaves that demand a high level of detail to render well. Images that work well with soft focus (typically glamourous/romantic images) can get by with fewer source pixels as well. So if you intend to produce wall-sized landscape prints that need to look good at arms length or closer, Michael's P45 kit might be the bare minimum unless you want to get into stitching. But if you're primarily doing soft-focus glamour portraits, a 6MP DSLR like the 10D might be more than adequate.
Lens and technique are also very important. A poor lens, improper focus, or small amounts of camera shake can "dumb down" a 12MP image to the 6MP level or below pretty easily. The more megapixels you have, the easier it is to fail to use them to their full potential. If you upgrade the camera, but fail to obtain lenses that can resolve as much detail as the sensor, you're mostly wasting your money. The same is true of cheap, flimsy tripods (save your money and buy a good one the first time around; it's much cheaper in the long run) and your own skill level. More megapixels demand greater focus precision, smaller apertures (DOF gets narrower because higher sensor resolution decreases the capture circle of confusion), and faster shutter speeds (motion blur at a given shutter speed becomes more apparent as the overall capture resolution increases). If you can't get a sharp image with a 10D, don't bother upgrading until you've mastered photography to the point you can, unless the problem is due to the equipment itself.
My personal rule of thumb is that most technically well-executed images from my 1Ds (11MP) can go up to about 24x36" before they start falling apart under close inspection. For the 1D-MkII (8.2MP), the limit is about 20x30", and for the 10D, it's somewhere around 16x24". But again, that's not set in stone, and varies considerably depending on image content and capture technique. I've shot many 1Ds frames that start falling apart at 5x7" because I screwed up some technical thing while shooting. YMMV.