Now, if I take that same image (making sure that Resampling is turned off in the Image Size dialog box) and change the dpi to your request of 300 dpi, the software will update the numbers in the Image Size and Resize boxes to about 8x7 inches. While that answers your question specifically, I don't want to stop there.
If you're asking about 300 dpi because you've heard that's the standard output for printing, you might want to consider some other numbers. Try experimenting with values from 160 dpi to 260 dpi to see what printers can do with your image. The technology inkjet printers use today offers superb results at 200 dpi that easily match the old 300 dpi needed by printers from a few years ago.
Getting back to my example, a 180 dpi resize of the 72 dpi image will produce dimensions of about 14x12 inches, which is easily resized to the 11x17 inches you mention by interpolating, or resampling, the original file. Digital camera image files resize extremely well. I've seen 11x17-inch prints from 4-megapixel cameras that rival a traditional film print. When you interpolate, however, you do need to do some slight sharpening.
If you become familiar with how resizing works with your image-editing program, you won't have to write me a letter when you're investigating an 8-megapixel camera (not that I don't like letters!). Just open up your software and create a new document that contains 8 megapixels, then start adjusting the dpi to see what kind of image dimensions you end up with.