Well.....with a laptop, you're locked into the RAM your motherboard will support. Look it up and buy it, not much else to discuss

One thing to consider is whether you already have a stick of RAM (of any size) in your expansion slot. Open it up and check, because you may be doing subtraction before addition...like remove a 512Mb stick and replace with with a 2Gb stick for a net gain of 1.5Gb.
With WinXP and no gaming, my gut tells me you're probably fine with 1Gb...why do you think you need more?
Unless you're running out and your machine is swapping from disk (which would make your computer come to a crawl) adding more RAM isn't going to boost your performance really.