Looking to increase my RAM

I dont know anything about the internal workings of a computer but I think it might be time to upgrade my memory...I have 256MB...and programs have been opening and running slow. And I was having problems burning a DVD and someone had said my Physical Memory is down to nothing..

I went to BestBuy.com and there are over 20 products to choose. From $30 to $100 for 1Gig.

Can you tell which would be a good investment?

I have a HP Pavillion Desktop with XP that is mostly used for backing up DVD's, downloading with torrents, and of coarse the internet.

Can anyone point me in the right direction

 

Pavilion 515n...I think thats it, thats what it says on the front

 

This would work:
pqi POWER Series 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 266 (PC 2100) Dual Channel Kit Desktop Memory - $50.99+$4.99 shipping from Newegg.

http://www.newegg.com/Product/Product.aspx?Item=N82E16820141218

Based on these specifications:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00006810&lc=en&cc=us&dlc=en&product=307585

 

Hey I just looked at those specs for the computer....is that saying I can't buy one 1GB replacement? I have to buy two 512MB if I wanted the max?

 

Your computer would run smoother with 2x 512MB sticks (dual channel) rather than just 1x 1GB stick.

 

OK I ordered the dual 512MB from NewEgg

Thanks guys