the old athlon xp 2500+(and similar models)processors are very good ,although around 3 years old they still are good solid chips,i still have a athlon p.c somewere in the house
first mate you need to run the crucial online utility(http://www.crucial.com/),this will scan your system and return a report with what type of memory modules to buy and also the make of your motherboard,it also lets you know what type of graphic card you need to buy
2gb of memory isnt going to come cheap,you are probally looking at around £150 and over,depending on the make you go for
all makes are ok,for example,the kingston value range are sometimes critisised,but i use them and there very reliable,however if you want gaming memory a make like O.C.Z would be a good purchase,but these are very expensive,other makes like corsair and even crucials own make are also very good
here are two of my recommendations,one being a value model and other being a power model
value memory...http://www.overclockers.co.uk/showproduct.php?prodid=MY-004-OK&groupid=701&catid=8&subcat=557
power memory...http://www.overclockers.co.uk/showproduct.php?prodid=MY-093-CR&groupid=701&catid=8&subcat=817
as you can see the website overclockers is very cheap,i get alot of p.c gear from there and find it good quality,especially the memory and graphic cards
good luck mate