The "double the size" thing is an approximation at best. I just did a quick check of 1D-MkII RAWs on my laptop, and they vary from 5,687KB to 13,314KB, depending on ISO and subject matter. The average file size (from a sample of 2656 images shot in a wide variety of conditions) was 8689KB. A 2GB card should hold about 240 RAWs. I'd simply adjust the JPEG compression setting to make the JPEGs a bit smaller if a 2:1 correlation in file size is important to you. Personally, I don't bother with JPEGs; I shoot RAW and back up the RAW files.