Found this while reading about 30D:
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Canon EOS 30D/20D or EOS 5D?
Whether you go for a 30D/20D or a 5D depends a lot on what you shoot and how large a print you want to make. If you don't need the 5fps of the 30D/20D and if ultimate image quality is your primary concern, the Canon EOS 5D is an excellent camera and is certainly capable of yielding higher quality images than the EOS 30D/20D. It's only bettered by the EOS 30D/20D if you're a wildlife shooter working at the limits of your telephoto lens. If you need to crop the EOS 5D images because your lens isn't long enough (and for wildlife shooters, no lens is really long enough most of the time!), then the EOS 30D/20D should yield better images due to the higher pixel density. However if you're shooting portraits or landscapes, or anything else where you aren't limited by the maximum focal length of your lens, and you are making large prints, then the full frame sensor and higher pixel count of the EOS 5D will yield better images, though it will cost you around $1600 more. For the average shooter, making prints from uncropped images no larger than 11x14, it might be difficult to tell much difference between the results from the EOS 5D and EOS 30D/20D, given the use of equal quality lenses. However for larger prints or prints from images which have been cropped, the larger sensor and additional pixels of the EOS 5D can yield higher quality images.
Another advantage of the EOS 5D is that, being full frame, it allows full frame lenses like the EF 16-35/2.8L their full wideangle potential. Those worried about the long term value of EF-S series lenses for APS-C sized sensors (like those in the 20D/30D) if Canon eventually go all "full frame" won't have that concern if they buy full frame lenses and own an EOS 5D.
taken from:
http://photo.net/equipment/canon/20D...D_vs_D200.html
Some very good comparison on the site. I do see the 5D has a bigger (larger) buffer size as well but no AF assist.