If you've still got the manual for your PC, it should say what RAM goes with the motherboard. Be warned that sometimes there are surprises - certain combinations of RAM sizes/types sometimes won't work, depending on the board.
Gurney's advice to take the RAM with you to the shop is good. As John's says about earthing yourself though, if you don't take good care of it then you're likely to kill it in transit. If you have an anti-static bag you can put it in, that'd be great, but most people don't. If you want to just take the RAM to the shop, your best bet is probably to put it in a metal box when you take it to the shops - that'll ensure any static charge on you is dissipated equally round the box so the RAM inside doesn't get zapped. It's not ideal, but it's better than any of the readily-available alternatives - a plastic box will build up static charges across its surface (especially if it's in the pocket of some artificial-fibre clothing) which is not good news for your RAM. The absolute safest way though is to leave the RAM in the PC, take the beige box (or black box, these days :-) to the shop, and let them open it up, find some suitable RAM, and check that it works afterwards.
As other people have said though, this is more likely a hard disk problem than a RAM problem, unless (a) you're trying to run a lot of things at the same time, or (b) your "music" is multi-track recordings. Clean up the IE cache, empty the "Recycle" bin, remove any files from the desktop (only *ever* put shortcuts on the desktop, never files, because that puts a big hit on your PC's startup time), and defrag the hard drive. Any or all of those might help you. It might also be worth running anti-virus and/or spyware detectors over your entire hard drive, because those can also slug your system and aren't the best for your PC's health anyway.