Ram upgrade help

I'm looking to upgrade my ram (Something I've never done before). My specs are located here:

http://forumz.tomshardware.com/hardware/Video-Card-Help...

Any advice? (where to buy, price, things to watch for, etc)
I have: Physical Memory : 512 MB (2 x 256 DDR-SDRAM )

Should I purchase 1 gig, or 2 more 256 cards? Do I need to look for anything else - can I just purchase "DDR-SDRAM" or do I need to pay attention to other specs?

Thanks.