I would like to upgrade my ram and instaed of buying 2 512 could i be able to get a 1 gig stick?

my computer is a hp pavilion a 250 eÂÂ! with a AMD athlon xp 2600 + 2.08ghz.
the max ram compacity is 1 gig.

Do i have to buy 2 512 or can i just buy a 1 gig stick and have it work the same way?

As far as I can gather, your system has an nForce 2 chipset that is capable of Symmetric Dual Channel (read this thread for info).

To take advantage of the performance boost Dual Channel offers, you must use a pair of matched modules; this means 2*512 MB is better than 1*1 GB.