Monitors & Memory - some advice
Hard to beat the Samsung Syncmaster displays in terms of excellent value and excellent image quality. To get the most out of it or any monitor you will want to invest in a monitor calibration device like the Spyder.
8GB of RAM will help with Photoshop CS4. More will be beneficial with CS5 which should be able to provide 64-bit support and address more memory than CS4 can do on the Mac. Just be sure to get RAM with heat sinks and get one of the name brand modules like Crucial or OWL. Best to balance RAM on your computer's two memory banks and have 4GB on each and not 8GB on one only.