It should say on the label on the HDD, but i'm sure you can find a tool on the internet which can find out for you without you having to shut down your PC and open it.
And upgrading your processor...well, it processes data, so it'd improve stuff such as video-editing, encoding, gaming, video-playing, music-playing, opening programs etc. It'll also multi-task better, with sufficient RAM (at least 512MB)
Extra RAM will allow you to have more programs running in the background. For example, my laptop only has 240MB of RAM available, so i don't have antispyware programs running in the background, and i use webmail instead of an e-mail client. Plus, i only use MSNM on it, instead of YIM and AIM, as they take up extra RAM.
HDD rpm matters, cause you'll be able to perform malware scans faster and have better peformance when accessing the hard drive with multiple programs e.g. playing a video whilst installing a program. I know, cause my desktop with a 7,200rpm HDD can scan for viruses a lot faster than my 5,400rpm HDD on my laptop (both using AVG Free).
