AS SSD might be a reasonable one.
Do not be much swayed by vendor synthetic SSD benchmarks.
They are done with apps that push the SSD to it's maximum using queue lengths of 30 or so.
Most desktop users will do one or two things at a time, so they will see queue lengths of one or two.
What really counts is the response times, particularly for small random I/O. That is what the os does mostly.
For that, the response times of current SSD's are remarkably similar. And quick. They will be 50X faster than a hard drive.
In sequential operations, they will be 2x faster than a hard drive, perhaps 3x if you have a sata3 interface.
Larger SSD's are preferable. They have more nand chips that can be accessed in parallel. Sort of an internal raid-0 if you will.
Also, a SSD will slow down as it approaches full. That is because it will have a harder time finding free nand blocks to do an update without a read/write operation.