I second the Alienbees monolight mentioned above. I've heard great things about them, and seen great results.

Photoblogger Justin Oulette uses one with a Hasselblad, and the results are beautiful.

The prices on the AB monolights are affordable, you'll prolly spend about 500 on a system (with umbrella, stand, and softbox...). Well worth the cost from what I've seen.

Hope that helps!