The thing you should be looking for is not so much how many megapixels the camera can claim but rather how close can it focus, and how small can the lens aperture get down to. The megapixels basically determine how large of a print can be produced with acceptable results ( the 2.1 mp is good to up to 4x6, the 3.2 mp up to 5x7, the 4 mp up to 8x10, etc.). The critical factor in your question is how sharp of an image can you get from a camera... for digital camera this is determined by the sharpness of the lens, the ability to focus, the ability of the CCD (charge couple device, the "film" of the digital camera) to resolve the image, the embedded software, etc. I would look to getting a medium size (3.2 to 5 mp) camera with the best glass and software; which means Canon, Nikon, Panasonic, etc. The second thing I would pay attention to is the ability to control the speed and aperture. The aperture is critical for sharpness because the smaller the opening of the lens to admit light (the f stop), the tighter the focus, everything else being equal. Shooting at a small aperture means the shutter has to stay open longer, which brings me to the best single item for improving model photos... a good tripod. The tripod holds the camera steady, which is absolutely critical for getting the maximum sharpness for this type of photography.
To sum up; look for a digital camera that allows you to have some control over the speed and aperture, can focus close enough to be useful, and is comfortable to handle and use. Plunk it on a tripod, get a couple of lamps (even clamp lamps from a DIY store), and you should be good to go. One thing I really like about digital photography is that I can take dozens of shots if I want to, keep one or two that I like, and delete the rest, over and over for basically the cost of the camera and batteries (I recommend getting a camera that uses AA's and using rechargeables).