If you want to go with a cassette larger than a 30, go with an MTB rear derailer. Road derailers are rated for only 27 or 28 (but they can often be coaxed to run OK on a 30). Earlier this year I put a Shimano Deore RD-M510 MTB "9-speed" rear derailer on my commuting/touring bike and I run 7-speed on that rig. It's been working great and it was only $22.
Some Shimano MTB rear derailers are low-normal (Shimano calls them RapidRise). I've read they are supposed to shift better but I've never tried one. I think the Deore LX RD-M580 is low-normal.
Last edited by Gonzo Bob; 12-18-09 at 12:57 PM.