I already have the OCZ agility 4 256gb. Which hard drive would you recommend? I prefer that it be around 1 tb (or more) but I don't want to go over $100. If it's beneficial enough to get a $100 hard drive instead of a $70 one, I will (but prefer not to).
Also, since I will have both a SSD and HD, what goes on what? From what I know so far, I think games go on the SSD and everything else goes on the HD. Is there anything else I should know?
OS and the games you are currently playing go on the SSD. You can always move them to the hard drive later if not playing and vice versa, Basically anything you want to load fast you want on the SSD, (game maps etc).
As far as the Hard drive. Since it's only going to be storage get a 1 tb 7200 rpm blue (WD thats my opinion) Seagate is a good company too. But i would stick with WD or Seagate. I say get a blue because you won't need the extra performance of a black since it's basically only storage. The one you picked is fine.
The Seagate drive that you linked will be fine for your purposes and the 64 mb cache and sata3 also make it a good quality performing drive.
Since you have a 256gb SSD for the OS drive , you have a good size drive that will hold the OS and quite a few games and the remainder of your files , such as music , photos and misc. programs and any games that your currently not playing can go on the secondary drive. You wouldn't necessarily have to move a game to the OS drive to play it if you decided to play an old game , it will play just fine because the OS is on the SSD along with your browser.
It's important to keep the SSD below 80% full capacity because as it becomes full it slows down in performance. Also do not constantly run benchmarks to see how fast it is or defrag it often or on any type of schedule.