I'm gaming primary I currently have a 750GB SG and it gets the job done I don't find that I'm running out of room but more won't hurt me, the real question is would an SSD be noticeable better then a VelociRaptor for gaming?
I have support for SATA III so I would plan on getting a HDD that supports it also (Vertex 3 etc)
The velociraptors were good in their time. Primarily because the 10k rotation gave them better random access time. That is what the OS mostly does. Sequential throughput was not significantly better than the more modern 1tb drives. That is because the 1tb drives are denser, and can transfer more data per revolution..
For games, you are probably more interested in sequential transfer, primarily for level loading.
The newer gen3 ssd's will be sometimes limited by a sata 2 interface. If you have sata3, look at one of the new gen3 drives. I would look at the Intel 510 drives. They are particularly good at data transfer, perhaps twice what a conventional hard drive can do. The Intel drives have had a lower return rate in the past, and I would trust them a bit more for a trouble free product.http://www.behardware.com/articles/810-6/components-returns-rates.html