I'm in the process of doing that myself. I just bought a 240GB Mercury Extreme Pro SSD new, off of eBay, for $400. Same as the one here:
http://eshop.macsales.com/shop/internal_storage/Mercury_Extreme_SSD_Sandforce/Solid_State_Pro
I'm going to try to do a complete backup of my current hard drive onto the new drive via SuperDuper, and then make the swap and hope it starts up with everything in place. We'll see.