I would look for a SSD with trim support. That would include the Intel X25-m, and there may be some others. Trim is important to be able to tell the drive that space is available without having to erase it. I think Kingston may have such a 40gb drive which would be good for an OS.
SLC is a better performer, but much more expensive than MLC. Products are improving rapidly, and prices are coming down. Research carefully.
Intel X25-M 80 or 160gb (gen 2) is a safe bet and a good performer now.