Image stabilization is important to me, however, the E-420 body only is about 13.7 oz. and the E-520 body-only is 16.8 oz...quite a difference. I have arthritis in my neck and shoulder due to many years of carrying camera and equipment...which is the reason the E-420 sounds better for me. That, of course, could change - - as I'm not purchasing a new camera quite yet...and am hopeful that one might be produced that has all my qualifications...maybe next year? or perhaps another camera (not Olympus) will fit my bill...
If size is very important to you, the micro 4/3s cameras should be hitting the market next year, with the panasonic g1 and other olympus models, and even if they dont have a macro lens, you will be able to use regular 4/3s lenses there.
If you want something sooner, the 420 plus the 35mm macro or the 105 sigma (if you want a longer macro) are a good value and probably the lightest thing out there out now. Id even suggest the 70-300 as a cheap, versatile almost macro lens, but that would probably make it all too heavy.