You have 4 slots so that you can use combinations of different size dimms
2Gb is good enough for almost any situation (unless you are running 64bit vista and editing movies)
.... depending what max the mobo supports ... (either check the manual or google cpuz - download and run it to id your mobo
then check the makers site to see what it supports)
http://www.microsoft.com/whdc/system/platform/ server/PAE/PAEmem.mspx
but if it's 4Gb
you can fit 2x2Gb or (cheaper option) 4x1Gb - or make do with (even cheaper) 4x512Mb
in your case 2Gb - so you could have started with 512Mb ... and upgraded to 2Gb as and when convenient
however ... dimms like to work in pairs ... so 2x1gb is optimum in your case