MoBo confusion

[Fallschirmjager]

My new system is almost ready to go. But selecting the MoBo about has me pulling my hair out.

Here are some basics.
I will need a socket 939 board, I will also need at least two free PCI slots for a sound card and wireless network adapter.

The cornerstone of my system will be two Geforce 7800 GTX cards from BFG
http://www.newegg.com/Product/Product.asp?Item=N82E16814143043

They arrived today and I was killing time at work reading over the product information.
One bit of confusion arose as I was reading.


I will quote it directly for clarity.
A vacant PCI Express slot adjacent the PCI Express x16 slot. This board occupies two slots: The x16 graphics slot and the one next to it

Two slots?
Most Mobos I see have two x16 slots and two 1x PCIe slots

Which two slots does this take? Does one card take both 16x slots?
Surely not...
Or does one card take a 16x slot and the 1x slot next to it?

This is really driving me crazy

Even if I get this thing working I will have used every PCI slot in my system (I hope the PSW I purchased can take all this abuse...)
If I ever decide to expand to another PCI based card I am screwed

So I see several boards that I could go with.
I have been an ASUS user for years and love their boards. But I would not be adverse to changing brands.
Abit has a model which I like, the Fatality
I also see that Asus has a A8N deluxe model. But it is always out of stock. It states that it runs both SLi cards at true 16x mode.
What exactly does this mean? What difference in performance will this make?
The cards state they have 24 pixel pipelines but the PCIe slots are rated at 16x. Are these two numbers different things?


I know this is also of questions, but the MOBO is the final piece to my new system and I want to select the perfect one.
Thanks in advance for any questions.


One final thing. If at all possible I would like a board that can run AMD 64/64 FX and 64 X2 cpus.
And cost is not an issue.


Edit:
I have the unhappy task of trying to build a system that is going to be used for both work and games as well as watching movies.
My work involves lots and lots of spreadsheets being crunched usualy while having a voice or video conference going on at the same time.
I decided upon the AMD 4000+ CPU to go in the system.
I plan on overclocking it 10-13%

 

[Crashman]

No, it doesn't require a second slot. It requires you to leave the next slot down vacant. That's because the cooler is so large that it prevents mounting a card in the next slot down.

Some boards are missing a slot under the x16 graphics card slot, that's because the company knows you won't be putting a card there anyway.

Most SLI boards allow you to use one slot at x16 or two slots at x8. There is not a performance difference with these cards between x16 and x8, but looking towards more powerful future graphics cards, the latest chipset allows x16 on two slots. Do you plan on using two graphics cards?

 

[Fallschirmjager]

Is the next slot down a PCIe 1x slot? (in most cases)


I will be using two BFG Geforce 7900 GTX cards

 

[Crashman]

The next slot down is usually either empty or a PCI-E x1 slot. Occasionally it's a PCI slot, depending on the board. With this graphics card, the type of slot under the card doesn't matter, they just want you to know you won't be able to use the slot that's covered up by the large cooler.

 

[Fallschirmjager]

So I will need to avoid boards that have PCI slots right next to PCIe 16x slots.
I need at least two free PCI slots

 

[Crashman]

I've seen boards that had two x16 slots and three PCI slots.