infernalchicken :
Do I need to install the os on the ssd?
How reliable is the Seagate hard drive?
Do I need a cooling system?
Nothing wrong with asking questions
The main advantage of having an SSD is that they run much faster than a mechanical disc drive. You can install the os on either but if you have an ssd you would be mad not to put the os on that
The os will run and boot way quicker. With a 120gb drive you could probably put all your programs on there too, meaning they would all load quicker. They don't provide too much benefit in gaming,apart from the game will load quicker if its on the ssd. So its really up to you whether you think you want one. I was undecided before I got mine but so glad I did - the difference is crazy!
The seagate barracuda series are pretty well recommended and should be fine. If you have the ssd i would just use the seagate for films/music etc.
As for cooling, i think you should be ok with the stock intel cooler. They arent as good as aftermarket coolers but i doubt you will need it. If you decide you want a cooler cpu you can always upgrade the cooler later on - the CoolerMaster Hyper 212+ is a great value cooler (£25-£30 here in the UK). I would recommend downloading CoreTemp if you want to keep an eye on your cpu temps.
Dan