I think you are pretty much limited to either a Fanatec wheel
http://www.fanatec.de/ or Madcatz. As the MS wheel is discontinued, I believe.
Fanatec is the best wheel in the long run, more expensive by far, but a really good wheel, some actual users are on this forum that can comment.
As for the Madcatz, its cheaper but I think those wheels lack Force Feedback.
Coming from me, a Intermediate Sim Race fan, Force Feedback is a must if you are going to invest in a wheel.
Also, I see Fanatec is going to release a Forza 4 wheel (2 models)
http://www.engadget.com/2011/06/09/forza-motorsport-csr-and-csr-elite-wheel-and-pedals-hands-on/ so if you have a few hundred $ to spend, wait for that one. But I know some people can't justify spending that kind of cash on a race wheel, so whatever is in your budget.
This scenario you are in is primarily why I bought a PS3, they have much better wheel support and Logitech makes inexpensive wheels that really make the experience.
Let's hope MS comes around!