In your case, I would say upgrade the body first. First off, I own a 20D,and have since it was the hot,new Canon. I shot it as recently as two nights ago. I still use it for casual snaps and flash work where I want modest file size, and a small,light body and need only moderate ISO settings. But the fact is, the 20D has a lousy, small LCD review screen, a very loud, tinny-sounding shutter that is exceptionally annoying (think aluminum softball bat hitting softball: TINK! TINK! TINK!), and pretty poor higher-ISO performance. The 20D takes a big performance hit moving from ISO 400 to 800, and the hit moving to 1600 is really significant on the 20D. It has no fractional, incremental ISO levels either...
The biggest handicap I think is the poor LCD screen when there's a need to review a shot for critical focus ascertainment...the LCD on the 20D is pretty lame. All in all, there are two dozen or more Canon and Nikon cameras that out-perform the 20D, especially at higher ISO levels. I think getting a significantly better-performing body would be a good option for you. I honestly think the T1i or T2i would out-perform the 20D for you.