i have a ocz agility 3, 250gb and i paid less then $200, so SSD's are quite affordable right now, just make sure that before you install it, you upgrade the firmware on the drive to the latest version.
i had my os installed on it to start, but quickly filled it up, after my os was installed, and diablo 3, WoW, SWTOR, and TF2 where copied over, and i almost had no space left, so since i had some hardware issues i reinstalled and used my normal HDD as my windows drive and everything that requires speed i put on the SSD so all the games, and whatever programs i need, like i have Visual Studio installed on the SSD and this solution has worked out well for me.
on the flipside you could build a machine with 64gb of ram, and use 32gb as a ram drive, and copy D3 to it, then you dont need the SSD, lol