Don't fight it, buy what states it "supports". I can only offer that as if it doesn't say otherwise, you're on risky ground. However, in your case quite possibility when released the AM2 class was offered certain heatsinks, that the AM3 class not being offered yet didn't cross-reference for its usage. Use your judgment if that's the case, otherwise what I wrote above applies in all good common sense. You are risking the health of a cpu and in AMD cpus, that's real important, they don't tolerate high temps well at all.
This is the heatsink I have: http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2414550&CatId=1187
I know it doesn't say it supports AM3, but it sure looks like it would. Should I risk it, or buy a new heatsink?