If you want to put games on there as well plug leave room for other apps and since they are cheap i wold get a 240-250GB. They rang from 90 bucks to 150 bucks depending on make and brand. Samsung is a good high end brand. Crucial (Which i have an older M500 240 in my gaming and media server) are a little slower on the read and write but i will tell you this. YOU WILL NOT NOTICE A DIFFERENCE. When it comes to Boot up, loading, ect most of the time anything that can read over 400 MBps you won't see a huge difference in. Even putting a SATA III SSD on a SATA II connector I still don't see a difference in windows boot up time. The big difference is write which unless you are doing a lot of like Video Editing or moving big files around all the time you won't see any difference there either.
But a 240-250 GB is a good size for windows, a few games, ect. I would just use the HDD's as a Cold Storage (or a place where once you put stuff there all you really do is view them, like movies, music ect). You can put games there as well but you will notice a bigger difference in load time on these newer games.