Yeah steam is pretty happy having multiple game locations. You can even move games really easily without having to redownload them. In that respect, you might get away with a 500gb drive easily as you could use this for games that most need it, or your most played, keeping the rest on the other drive.
You only mention steam games: I thoroughly recommend putting windows on the SSD too, if it isn't already. The benefit you'll see in boot times and general responsiveness is huge.
Also +1 to the evo. That said, SSDs are all pretty close in performance (especially compared to a regular HDD) so you might consider a nice offer this weekend. Just check some reviews first to make sure its decent.