So yeah. Should I get one larger hdd and put my OS, music, games and EVRYTHING on it or go with a seperate smaller drive for the OS and one for all the other stuff.
Would there be a performance difference with either of these senreios? Any recomended drives for either case? If I should go with just one drive, should I partision it?
There would be no performance difference. If both HDD's are equally fast, than it won't make a difference if you put it on one bigger or two smaller HDD's. If you want great performance, than go with an SSD. You could put your OS and your basic programs on the SSD, and the bulk data on the HDD. In this way your computer will boot up blazingly fast, and you will have some extra memory.
Your going to have pretty much everyone tell you to get an SSD, though you have to be mindful that atm ssds dont last anywhere near as long as a normal HDD. I personally would recommend getting a Samsung drive. Samsung 840 or 840 pro. They're amazingly quick as a boot drive and plus they're reasonably priced. After that you should get a seagate, though you already have. Baracudas are amazing storage drives and will last for an extremely long time.