Gunbei is absolutely correct. Megapixels do not tell the whole story and are certainly NOT an indication of image quality necessarily. There are cameras that produce a with 4 megapixel ccds that will produce consistently better images than cameras producing double that. The biggest jump in image quality you are likely to invest in in the prosumer camera is jumping up to the (physically) larger CCD of most of the (prosumer and professional) SLR cameras, and taking advantage of the RAW file capabilities of those cameras (much larger files which require conversion to use as jpegs or tiffs). Again, even there, numbers don't tell the whole story...use your eyes, consult the reviews. The numbers game, especially megapixels, means about as much in the consumer/prosumer camera market as it does in high-end audio It is used in the consumer market as a marketting gimmick. Definitely consult the review sites and magazines that have already been recommendded by Gunbei, Prpixel and others. If you are limiting your blow-up size to the typical 8X10 inches or so, as Gunbei suggests a good 3-4 megapixel camera can do that quite well.
To clarify a few points in Prpixels post; Pro SLR cameras are available in 35mm SLR body-style from around 4 megapixels to 14. As an example; The 4 Megapixel Nikon D2H is a remarkable camera, as is the 11 Megapixel Canon 1DS. Each tool has it's advantages, and each one produces amazing images for the state of the art today. Both have their advantages and drawbacks. The digital backs used on Hasselblad and other medium format cameras have been around a long while, and just like the consumer market are getting better and better. They are still quite expensive $10-20K on average, and up until recently most had to be tied to a laptop in order to shoot. There are also digital backs available to shoot with large format 4x5 inch cameras. If you shoot a lot of pictures, digital is certainly the cheapest way to go. Keep in mind on digital cards you dump the files onto your computer, wipe the card clean again and shoot over and over. Film still has an edge in overall quality, but the average person will not take advantage and may not even notice or care about those diffferences.
Marco