Which Hard Drive to choose - Pitfalls

Hi Guys, I currently have an AMD Sempron 2400 & 224mb ram, not very satisfactory, can't trawl the web and run Outlook at the same time without running out of just about everything, system rescources running dangerousy low etc:

I have limited pc skills and don't know the best way to go, probably the first would be is scrap it but can't afford to so I will have to try to upgrade as cheaply as possible.

Can anyone tell me what i need to look out for and what would be the compatible HD's for this system and would it be adviseable to increase memory to 512mb ram ?

Many thanks guy's.

 

Ram would be the first thing to upgrade, use this as a guide,

http://www.crucial.com/