You need to determine what speed that RAM is. DDR and SDRAM are essentially the same thing. If there is a part number you could try googling and see what comes up.
You can go to http://www.crucial.com
and let them scan your computer. It will tell you what is in there and what you need.
Alternatively, you can check your owner's manual.
I paid $50 for 2 1-gig sticks from New Egg. Some computers can use Dual Channel memory. The crucial scan should tell you that also.