3.5 hdd or 2.5 hdd??

Hi I'm wanting to upgrade my hard drive from 160gb to 1tb but looking at the hard drives online , they are selling 3.5 and 2.5 hard drives. I have no idea what my old one is, and if i buy the wrong one will it still fit? and whats the difference between 3.5 and 2.5??
Also I want to upgrade my cpu from a celeron 2 gz dual core to a i3 dual core 3.3ghz processor, is this possible? my motherboard is a Asus p5n73-am

 

In the world of mechanical hard drives, 2.5" drives are used in laptop computers, and 3.5" drives are used in desktop computers.

As far as a CPU replacement, you're screwed. Your MB uses an LGA 775 socket. There hasn't been a cpu made for that socket in over 2 years. If you want a new CPU, you'll have to get a new MB and new ram also.

 

I'm glad I asked about the cpu, !I suppose I could try to overclock the one I've got, but never done it before, hope I don't cook it, but then if I do the wife might let me buy a new motherboard and cpu.he he