2008 Macbook pro, 10.6.8, upgraded to 6G RAM

I have a 2008 Macbook pro 10.6.8 & I upgraded to 6G RAM & new battery in 2013. Still, after it is on for an hour or so, the caps lock light starts blinking, fan whirs laboriously, laptop gets hot below the keyboard, then the mac eventually freezes up. If i need a new hard drive, I'm wondering which is best. I do a lot of , internet searching and movie streaming. I also do some photo & video editing & design. I want to keep this laptop for as long as I reasonably can.