Also just because your motherboard has the slots, doesn't mean it has the bandwidth to run all the cards at that speed.
Chances are you motherboard would only do 8x/4x at the same time.
If you crossfire, there will be performance gain.
Any video card can work in any PCI-E slot, even in a 1x slot but that would hinder performance considerably.
You can only crossfire a video card from the same series and from the same tier denoted by the first and second number on ATI graphics card numbers.
You should only crossfire cards of the same model because a faster card will downclock to match the speed of the slower one.
4x slots hinder performance. I agree with wa1 that you should just consider upgrading to a faster single card. My system is running CF 5850s on 8x/8x and it's fine they get up to 99% usage but at the same time my 3rd card, a dedicated GT240 for PhysX is on a 4x lane and I've never seen it over 83% usage tops. That's while running FluidMark.
You would see a benefit running 16x/4x or 8x/4x crossfire, but you won't be using all of that second card. So yeah, probably better to just buy a better single card... maybe sell your current one if you can (or if it's a suitable Nvidia card, use it as a dedicated PhysX just for fun)
EDIT: Just realised you did indeed say Xfire so I guess it's not an Nvidia card hehe.