Crucial click here should b able to tell you what memory you require.
To find out your mobo if you don't already have the manual in with the docs that came with your pc then the way I usually do it is to open the case and look for the maker and model number on the board and then google it for a manual.
This i'll tell you all the specs of ya board like what AGP slot it has eg 2x 4x which will help ya find ya graphics upgrade.
As far as the rest goes you should be able to fine a USB2 card for around £10-20, memory depends on what memory u need and how much u want to put in prob end up being around the £40-50 mark. Hard drives are pretty cheap now again depends on the size u want to put in and make looking on ebuyer click here u can pick up an 80gig seagate barracuda for about £30