PC or MAC?
You need the basics: Min 2gb mem, 2+ ghz, 100mb hard drive.
One thing that most people think they need is a fancy video card, but photoshop (with the exception of the new enhanced version) is 2D graphics, you don't really need HIGH END for that.
What you do need is a good LCD. High end Asus have really nice screens.
To see what a bad screen is look at this:
Picture from Dell and HP comparison
Budget no object?
Specifications
Apple $3,800.00
2.4GHz Intel Core 2 Duo
4GB 667 DDR2 - 2x2GB SO-DIMMs
160GB Serial ATA Drive @ 7200 rpm
SuperDrive 8x (DVD±R DL/DVD±RW/CD-RW)
MacBook Pro 17-inch High-Resolution Widescreen Display
Backlit Keyboard/Mac OS - U.S. English
Asus W2 Asus/Lamborghini VX2-AK00xG Ensemble Notebook Series
Dell makes one with a 20" screen, starts at about $3,000 and gots up over $7,000