The only difference between SSD and HDD for games is loading times. SSD is faster than HDD. SSD is more expensive than HDD, so people usually have an SSD as their boot drive and an HDD as storage.
The only time storage device speed affects games FPS is when you don't have enough physical memory (RAM) to hold the game data after it loads. If the game data is more than your RAM can contain, the remainder is loaded onto disk. In windows they call this virutal memory AKA paging AKA Swap. While paging, games slow down quite a bit, so you need to have enough RAM. If you have 4GB+ memory in your PC, you are probably OK.
I find SSDs more of a luxury than a requirement. If you want your computer to boot up fast, get a SSD. The loading times are really only decreased by .5 to 1 second for me. Also, the price per gigabyte is very high for SSDs. A 7,200 rpm hard drive is your best bet if you don't want to spend a ton on hard drives. Also, Sata 2 max speed is 3gb/s vs Sata 3 6gb/s. So sata 3 is usually in SSDs with 300 mb/s or higher.
An SSD has the most use as an OS and programs drive. It won't affect framerates in games, but as an OS drive your PC will feel much faster in normal usage.
Also Tom's just did a review on the hybrid drives, particularily the Seagate Momentus XT 750gb and I would suggest that as a very good option to go, with much better load times but also much cheaper and more storage room.