What is it about the 5D image quality you feel is letting you down? "Image quality" has many areas, but I find the original 5D to still produce excellent quality images when you aren't pushing it's limits. I have had a "classic" 5D since they first came out and a 5D mk3 for around 3 years, so I am well versed in the qualities of both.
I upgraded, primarily, for the better AF and other features that make the mk3 better for sports etc., which used to get handed off to a 40D. The mk3 is a solid all rounder and the 40D rarely gets used these days. However, the old 5D does still get used as the images have a certain indefinable something that makes them really good. So long as I don't need high ISO, fancy AF etc., I will often use the old 5D in preference to the new one for the better image quality in some areas. In particular I find the 5D records better skin tones than the mk3, not by much, but I do find that with studio portraiture the old 5D can produce better results than the mk3.
Either camera though produces great images, so I wouldn't switch for IQ / DR purposes unless you are pushing the limits of your camera and are unhappy with your images due to noise etc. Clearly in low light situations the much greater ISO capability of the mk3 will give better results at high ISO (particularly as the 5D tops out at 3200 so if you need more than that it really suffers).
If you are feeling limited by your current 5D, then an upgrade may be the answer (and probably is). However if you are happy with the results you are getting, and just feel that you would get something better with a newer camera, then you are probably better off investing in lenses. The classic still produces results that are good enough for national exhibitions.