Programming?? If so, get a monitor with the highest resolution you can afford unless you love scrolling and hitting the alt-tab combo.
Or get an operating system with workspaces. It gives you the capability of having multiple desktops that you can easily jump back and forth from. What I typically have is email/web in one space, edit sessions in one another, build and debug sessions in another, virtual machines in another.
Mac, Unix X/windows and Linux have them built in. Windows XP and Vista do not, but I read somewhere that there is software available to have a similar capability. Don't remember the cost, I think it might be free.