bilalwaseem :
dude....go for the ddr3....which is the latest version of ddr....and i good modern technology.....dont buy the ddr2 one....it is alittle bit old though....
the modern technology of ddr in graphics card is ddr5....
did you understood what i said
???
I understand what you said. You told him to buy DDR3 RAM even though the mobo he listed above only uses DDR2.
Ignore that comment maty0255.
If you want to add just one stick of memory - best to match it to the other one you have. Open the case and get the model number of the memory off the memory stick (you might have to pull it out - which is very easy) and go to the vendors site for a matching model number.
Just take care that before and when opening the case you observe common practices to avoid electrostatic discharge - see this article related to upgrading a video card for advice on what I am talking about:
http://www.upgradevideocards.com/install.html
Otherwise, to replace all RAM, you can go to the website of a major supplier, like say Crucial, and input your memoryboard number and get upgrade options , like this:
http://www.crucial.com/store/listparts.aspx?model=P5KPL-AM%20EPU&Cat=RAM
On this page you can buy 4 GB of the slower 800 MHz memory for $50 or faster 1066 memory for $80. Obviously faster is better, but depends on budget. Note that the "orange" memory has some advantages but works at a different voltage level that may require you to make some BIOS changes to make it work.