It is best to hold out as long as you can to get the biggest SSD you can at the cheapest price .... but since your HD is dying anyways.
Get the biggest you can afford from a quality manufacture. That is usually intel or samsung, though most of the early problems with SSDs are gone, you still don't want a drive failure. At least 180 gig for your OS and any common use programs. Also note, these drives need a lot of extra space as they move data around a lot to optimize drive life.
Sign up for neweggs daily deals emails. They always have some SSD on sale for a good amount off the normal price that won't be beat anywhere.