PC requirements for image editing vary with the application, i.e. for Web graphics the file size tends to be lower than for print. Typically Web graphics are displayed between 72dpi and 96dpi, whereas for print this could be in excess of 300dpi.
With Photoshop's multiple layers a working file size can easily exceed 50Mbytes, and as a general rule of thumb you should have three to five times this available as RAM. It is true that ALL graphics editing software is RAM hungry, with the multiple undo levels now available in Photoshop even more RAM is used, I would not recommend using less than 512Mbyte with Windows XP.
The monitor/graphics card combination must be capable of working to 24bit (True Colour) depth. I would not recommend less than a 17 inch monitor, if you can spare the desk space (and cash).
As Photoshop uses its own virtual memory (scratch disk) you should have at least 200Mbytes of free local disk space. Photoshop benefits from having a separate disk allocated as a scratch disk.
Image editing using a mouse has been (accurately IMHO) compared to being "like trying to draw with a bar of soap", so although not essential a graphics tablet cannot be recommended highly enough. Make sure that you get a pressure sensitive tablet such as one of the Wacom range. The size of the tablet is not that important, a 4x5-inch (A6) active area is fine, many people prefer to work with smaller tablets anyway. I now use a 9x12-inch Wacom Intuos tablet after upgrading from the A6 Wacom ArtPad II, on reflection I think the A5 Wacom Intuos would have been a better choice.
The Windows XP Pro SP 2 PC I currently use
|CPU||Intel Core 2 Duo E6700 (2.67GHz Conroe Core)|
|RAM||4GB DDR2-667 (PC2-5300)Â SDRAM|
|Hard Disks||Seagate Barracuda 250GB SATA-11 7200rpm 8MB cache|
|2x Western Digital Raptor 150GB SATA-II 10K 16MB cache|
|Monitor||21.3" Iiyama ProLite H540S TFT (working at 1600x1200)|
|Graphics Card||NVIDIA 7950GT|
|Motherboard||Asus P5W DH Deluxe|
|Tablet||Wacom Intuos 9x12|
|Scanner||Minolta Dimage Scan Elite 5400|
|4x WD1000FYPS 1TB drives - 1GB RAM|