Yes, try and add the same manufacturer's memory with exactly the same specs. If you can't find it, you will need to buy two new sticks (a dual channel pair) of DDR2 800. I would go with the later as it will avoid any compatibility problems.
And one more thing i am planning to install ati sapphire 6670 ddr5 to my comp!! Do i have to change my bios version after instalin the card???
My current BIOS version is 184.108.40.206.05!!!!