Get the largest screen you can afford. Get a CRT monitor, they're better for graphics and image work, no matter what anyone may tell you to the contrary.
Do NOT opt for the cheapest PC around - you'll be glad of plenty of processing power and as much RAM as you can manage. As Taran says, you'll also notice the benefit you get from a dedicated graphics card.
Finally, go for a broadband connection if it's avaliable and your budget can stretch to it. Once you start uploading sites to servers you'll really appreciate a fast connection.