Good and cheap SSD. Size matters not.
I got a Corsair Force 3 60gb for 30 euros brand new.
I'm using that same SSD in mine for OS and a couple of other progreams. It's not the best in the world, but read/write speeds are both around 500. You can also speed that up big time with a RAM cache.
I want one a cheap one to. But the cheapest i can find is the corsair nova 2 30 gigs
The nova 2 get terrible reviews. I would not recommend getting it.
Either sold out or way over that price :(
Samsung 830 or Crucial M4 should be pretty cheap.
Might just stick with a regular old HD. Maybe even get a Momentus XT :O