I found no problems at all installing multiple USB3 cards in multiple Dells 3-4 yr-Old (both using Win7). Slots were available but had to do some gerry-rigging on power (a cable or two to piggyback)
I'm a fan of the Startech USB 3 caddy for the drives and upgraded USB cables
I do have a question though about speed. I assume that the highest transfer speed is in the motherboard. If I use a USB 3 card and plumb the output to another hub (in this case my monitors USB hub) will I lose the speed advantage using that hub?