You are on the right direction i think but have a little approach mismatch.
The thing is when you just bring to your document it will look smaller because actually making something 300dpi instead of the standard for a computer monitor 72dpi is just reducing its size like four times. (dpi is dots per inch = pixels per inch but we all know that)
If you want good quality I recommend not using the standard photoshop resize tool but some professional software as Genuine Fractals for example.
First open the image in photoshop and from image size adjust it first to 300dpi click ok the go to image size once again and then enter the pixel dimensions that you need. And when performing both operations make sure when you click the button Auto on the right (at least in CS3) the quality is set to best.
It should work fine but for such a resize you’ll loose a looot of quality when using standard photoshop tools. They work just fine for a x2 resize though 🙂