workstation (ATI V7100) + gaming card (ATI 3870) in one case?

it should see both cards. thinking that you'll install
the drivers for both cards.

PCIe cards get some power through the bus.

are you trying to run 2 monitors, or to switch
back & forth between the 2 graphics cards,
using the same computer, and the same monitor ?

it sounds like you're talking one monitor. a KVM
switch will work for that.

As previously said, many cards get power through the bus... So I would be concerned that unplugging the power will cause more problems than just leaving both cards configured properly. You should be able to run two (or more) cards at the same time, both connected to separate monitors. Or the same monitor, for that matter. Many people have done the dual cards, and so long as you don't have driver issues, you should be fine.

I'd also be hesitant to be plugging/unplugging the power on the cards all the time due to wear/tear/flexing of the cards. Plus the hassle of taking the cover off, etc. Really, that that point, you're just as well off to remove the sucker entirely and plug in the other card.

My Viewsonic monitor has a DVI and VGA input, and a button on the front to switch. I use this as a poor man's KVM with my laptop and desktop. Long story, don't ask. You might be able to find one with dual DVI inputs, or find an adapter.

Clint