A few photos per board but id imagine pics to be 4' across... She says the printers will require 300dpi, im thinking my meagre 20D with its 8mpx isnt gonna hack this.
There are several issues here which are all muddled up together. Let's see whether we can unravel them.
Firstly, there's potential confusion between PPI (pixels per inch) and DPI (dots per inch). Strictly speaking they're not the same thing because most printers use several dots of ink to make up one pixel. But that's a technical detail which is mostly irrelevant, and when someone says DPI you can be pretty sure they probably mean PPI. If they really mean DPI in the technical sense it will be obvious from the context, and that's not the case here.
Secondly, the reason why they say they need 300 PPI [sic] is probably because the printer which they're going to use is capable of printing at 300 PPI. But it's immediately obvious, without even doing the maths, that a 4-foot print at 300 PPI is going to have many many more pixels than any camera can supply. That needn't matter. You're not going to be looking at a picture like that from right up close, so it's not necessary for every pixel to be unique. You could upscale an 8-megapixel image to (say) an 80-megapixel image, and that will still look fine as long as you're not too close to it. Many printers can handle a smaller input file by up-scaling it internally, but some might not be able to. In any case, it's usually possible to get better results if you do the up-scaling yourself. So you need to establish what they really need as an input file. Do they really need a full-size file or can their printer handle the up-scaling for you?
Thirdly, a 4-foot image is BIG. That's 48" by 32" if we stick with the usual 3x2 DSLR format, so at 300 PPI that's 14,400 pixels by 9,600 pixels for a total of 138 megapixels. (So even a full frame or medium format camera would still be way, way short.) As mentioned above, there's not necessarily anything wrong with up-scaling your images from 8 megapixels to 138 megapixels, but if they really need a 138-megapixel input image then you might need to think carefully about how you create it to retain the best quality.
