if you decide to add more ram make sure its the same as you already have in. its far better to have 2x512mb,2x1gb..e.c.t, than 1x. Dont mix sizez i.e 1x512 and 1x256. If you need to remove a ram stick 1st make sure you are earthed (Use an anti-static wrist band) or at the very least keep hold of a metal section of the tower.When you open the tower you should easily see the ram stick connected to the motherboard, there are two clips on either end that must be pushed downwards,the ram will then pop out and be released. once removed there will be a sticker on the stick of ram indicating what speed it is. you should look for 'pc...' or ddr.... or ddr2...' the numbers after this denote the speed of the ram. If you can buy a matching stick this will ensure compatability but it aint the end of the world if it isnt the same brand. Ebay is a good place to start as prices are reasonable. Oh and dont be tempted to buy 2nd hand, it may appear ok until your system is under load but then may throw up errors if its faulty(long after you have left good feedback)
Adding/changing ram is straightforward so dont be worried. Finally when you install the new ram you may have to push it back into the slot quite hard (this is normal) The best way is to push at either end of the stick untill the holding clips pop forward and clamp it into place.
Windows will then automatically allocate suitable pagefile size and virtual memory so no settings need to be altered.
Hope this helps
