Ierland or BMP have everything you need in the way of urethane bushings. I've heard they squeek but only when the engine is at low enough revs to let you hear it. The big issue is ease of installation. First you need to burn the old rubber out of there, then make a tool to press in the original rubber bushings. Urethane bushings are two piece so they're easier to install, no pressing required.
If you care, you can search the Roadfly.com or BMW FAQ archives for opinions.
I'd go urethane but I'm generally pretty lazy and shiftless.