I would still open the case up and check the following:
The number of sticks currently fitted and how many slots are spare
The ratings of your current RAM Is it PC2-5300 or PC2-6400? PC2-6400 is faster.
If you already have 2x PC2-6400 memory sticks, then you can order another matched pair and increase your RAM on top of your current RAM.
If you have PC2-5300 RAM I would recommend you replace them with new PC2-6400 RAM.
You should order memory in pairs (matched pairs), as otherwise they can't function in dual channel mode and it restricts performance of them. According to Crucial you have 4 slots, so 2 are likely being used by 2x 512mb RAM sticks, leaving 2 left.
The best place I find to order computer stuff here in the UK is from www.ebuyer.com I don't use anywhere else.