SLi vs CrossFire

Well here goes a long explanation.
Crossfire is two ATi Cards bridged together.
SLi is two NVidia cards bridged together.
Think of multi-card solutions as a single card, but instead of having two GPUs on a single PCB, like the GTX295 or the 4870X2, you have two separate PCB's and a 'connector' that bridges the two cards together.

The ability for the motherboard/drivers to recognize Crossfire/SLi is directly related to the Chipset manufacturer.

Let's assume you have an Intel X38 board. This board supports Crossfire ONLY, meaning you can only get support from bridged ATi cards on a X38 board.
If you install two Nvidia cards and put a bridge on them, the motherboard will recognize them as two separate cards, and the drivers will not allow SLi.

Now, let's assume you have a Nvidia 780i board. This is an SLi board, not a Crossfire board, so you can only get a viable multi-card solution by using Nvidia graphics cards. Adding two ATi cards will show up as two separate cards when booting up, and the motherboard does not allow the cards to be taken full advantage of. Attempting to start a game will only allow utilization of one card.

Here's the clincher though.
X58 based motherboards (Core i7) support BOTH SLi and Crossfire solutions; meaning you can plug in two of either brand and still have the motherboard and driver software recognize two cards and utilize a dual card configuration.

Hope that answered your question.

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If the motherboard uses a NVIDIA chipset set, then it is NVIDIA and supports SLI only.
If the motherboard uses an AMD chipset set, then it is AMD and supports Crossfire only.
If the motherboard uses an Intel chipset set, then it is Intel and supports Crossfire. SLI is supported as well with only on X58 or Skulltrail.
If the motherboard uses any other chipset, then it supports neither Crossfire or SLI.

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I think this is addressed to me.
You can tell which motherboard supports NVidia or ATI by the manufacturer.
If the board is based off of an Intel chipset (X38, P43, P45, X48, etc), then it will most likely support Crossfire only.
If it is based from a NVidia chipset (680i, 750i, 780, 790, etc), then it will only support SLi.
AMD Chipsets support Crossfire ONLY.
There are a few exceptions, Skulltrail and X58 being the two most prominent.

X58 does not support 'hybrid' SLi/Crossfire, as both Drivers and Operating systems do not allow this (WDDM in Vista only allows one graphics driver at a time to be functioning).
You can however, on most X58 boards, use either a Crossfire or SLi solution as a drop in upgrade, meaning you can use two 4870's or two GTX260's without a problem, and switch between the two if you feel like it.

Just make sure to read reviews and check the specs of the board, before you buy it.

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Correct, but if you are using Vista, they must use the same driver.
Meaning, no NVidia/ATI combos.
The easiest way to guesstimate if you are using the same driver is to make sure that the cards are of the same 'series', IE, 4350 as a discrete card, and 4870x2 as a gaming card. Both are of the 4000 Radeon series.

Under XP you can use both.
Under Windows 7, the expected outcome is WDDM 1.1, which you can use two display drivers.

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Not all i7 boards can do both, the cheapest MSI board only supports crossfire.

The SLI boards will have nForce in them (example EVGA nForce 790i Ultra SLi FTW) whilst crossfire is supported on the Intel chipsets (P35, P43, P45, X38, X48 and X58).

I'm not sure about the AMD boards

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Correct.

Like I said, the easiest way to know if you are using the same driver is to check the card series.

If using a 4870 and another 4870, you're fine.
If using a 4870 and a 4350, you are also fine, as they are of the same series and use the same driver.