Like is full power not being used on the PCI-E slot?
X16 or X8 refers to the number of data transmission lanes which determines the maximum transmission rate across the interface.
For all practical purposes, it does not matter if a slot is X8 or X16 with today's graphics cards.
A graphics card will perform marginally better if it can use all 16 lanes of a x16 slot. But, if you will be planning on sli, then X8/X8 is fine, and that, as a rule is all you get on modern z77 based motherboards. There are some high priced enthusiast motherboards that have added chips to give you more for triple and quad sli.
Now, I suggest it should be a moot point.
I think one should not be planning on sli up front if a single good card will do the job.
Here is my reasoning:
) How good do you really need to be?
A single GTX560 or 6870 can give you great performance at 1920 x 1200 in most games.
A single GTX560ti or 6950 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single 7970 or GTX680 is about as good as it gets.
Only if you are looking at triple monitor gaming, then sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards.
b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.
Your psu costs are less.
A GTX560ti needs a 450w psu, even a GTX580 only needs a 600w psu.
When you add another card to the mix, plan on adding 150-200w to your psu requirements.
A single more modern 28nm card like a 7970 or GTX680 needs only 550W.
Even the strongest GTX690 only needs 650w.
Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.
d) dual card support is dependent on the driver. Not all games can benefit from dual cards.
e) cf/sli up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.