Actually you may already have a built-in solution to your problem and not even know it. But first let's look over your situation a bit more closely as you haven't necessarily provided us with all the necessary information by which to provide you the best answer.
I'll put forth a scenario that may apply to your given situation. However, if it doesn't there is nothing stopping you from purchasing the necessary monitor, etc.
If you have a much more recent monitor (LCD type purchased within the past 3-years or so) it may have as inputs both the old standard VGA and the more recent DVI styles. If your Dell outputs video in VGA you may then connect the Dell to said monitor via the VGA cable. If you are purchasing a Mac Mini or the Power Mac you can output video from them via DVI to the DVI input on said monitor. The Mac Mini only outputs video via DVI (but does come with a VGA adapter) and while I cannot speak for the Power Mac I will guess that it outputs in both VGA and DVI.
Your monitor, since it can handle either input, should have a switch (a single switch or an option within its menu system) whereby you can switch back and forth between the two input types. Even if the ability to switch is buried inside the menu options it's still relatively quick and you don't need to buy anything else (like a KVM switch, which I believe start around $50).
So you see, you may already have everything you need to do exactly what you wish. If, however, you do not have a more modern monitor with this dual-input option then this would be a great time to modernize your monitor with a larger LCD type. Granted, spending $200-$300 for a new monitor is more expensive than a KVM switch, but you will certainly appreciate the increased real estate you gain back on your desk and equally appreciate the incredible workspace you find on a 19, 20, 22-inch or larger LCD monitor.