I did this just recently. SSDs are now under the €1 per GB price mark, which is what I considered the appropriate point to make the switchover, and you can quite easily buy 256GB for a very reasonable price. At that kind of cost I left the OS and program files on the SSD and moved the User Profile stuff to the regular HD; I also used junction points (via mklink) to link some of the folders from D to C, which works very well.

Personally I'd say that if you can afford it you should go for 256GB; it'll pay you back in the longer term from the increased performance and battery life.