here's what you need to do to find out the kind of RAM your system takes...
there are 2 ways:
1) open the system up and take out the RAM itself and look on the lable it should say what type of RAM it is (ex. 1GB DDR2 PC-5300)
2) if your not comfortable opening your system just yet download and install Belarc Advisor. run the program and it will have a section that tells you exactly what type of memory is in your system.
after you find out what type of memery you have its just a matter of goin to bestbuy or any other store that sells computer components or go online to sights like tigerdirect.com and buy the RAM..
To determine how much RAM you need i would say a minimum for running a windows xp system is 1gb if your a big gamer or do alot of photo editing with programs like photoshop or paintshop pro you should get atleast 2gb of RAM..
most 1gb sticks cost between $40-$150 depending on the brand
well hope this helped....