|There is no DDR300. The three most common are DDR266, DDR333, DDR400.|
DDR400 *should* be backwards compatible so if you currently have DDR333 & add a stick of DDR400, both sticks will run as DDR333. You could also get DDR333 to match what you have.
You might wanna enter your board model number into the Crucial Memory Advisor™ tool to see what they advise. And if you're in the US, shop around online.
Here's a few possibilities: