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circesey is correct, 1800 and 1900 series cards needed a Crossfire Master card and it connected to the second card with a Dongle cable between the two cards on the outside of the case. The did away with that with the 1950 cards.  
 
The 3870X2 sort of works like two 1900's but instead it has the Crossfire chip between the two GPU's on the one card and this chip is identical to the chip used on the 1900 Crossfire Master card.

1_3870X2 is like 2 cards attached to one so i am not wrong about that  
 
2_Thanks for that i will fix it  

Edited

I have to agree with Maziar on this one....it IS 2 cards since if it was just 2 gpus on one card they would not need video ram for each gpu...just one set of ram for both.....they are just crossfire on a stick. so its JUST like nvidia.

1 card
 
http://en.expreview.com/2007/12/16 [...] leak-pics/
 
http://www.quantum-force.net/tutorials/T000000006/

I think u misunderstood it
I meant HD 3870X2 itself is a single card but its actually 2 card attached to each other just like 7950GX2 which needs only one single PCI-E 16x to run

I disagree with Maz and nuke.  The 3870x2 has only one PCB, hence it is one card.  The 9800GX2 has two PCBs, so it is two cards.  You can argue against this all you want, but there is only one PCB.
 
In the grand scheme of things, who cares.  Be aware that each uses SLI/CF, and each card has its own benefits/draw backs.  Whether it is one card or two doesn't really matter that much.

And here is the final answer:
http://www.anandtech.com/video/showdoc.aspx?i=3209
 
AMD is the first out of the gates with the Radeon HD 3870 X2, based on what AMD is calling its R680 GPU. Despite the codename, the product name tells the entire story: the Radeon HD 3870 X2 is made up of two 3870s on a single card.
 

Let me fix the quote for you.
 

 
The 9800GX2 is two PCBs facing each other, bleeding heat into the same heatsink, with a fan running air over it for cooling.  Each PCB has a single GPU on it.  The 3870x2 is a single PCB, with two GPUs on them.  The one in the back has an aluminum cooler, with the GPU in the front has a copper one.  It does indeed have two 3870s on it, but they are facing the same direction, on the same PCB.  If you count "cards" based off of the number of PCBs, then the 9800GX2 is a dual card, while the 3870x2 is not.

If the things u say are true then why it says its actually 2 HD 3870 on a single card and also why in the tests when they use 2 HD 3870X2 then they call it QUAD-CROSSFIRE?

I do not count PCB's as a card each.

By that logic QuadFX was a quad core just like intels(and not 2 dual cores with all the downfalls people mentioned....even intel is 2 x dual cores anyway). since it was on the same board with 2 sockets and 2 memory controllers.

the X2 and GX2's do the same thing with one or two boards they still have 2 full cards(minus the video outputs..). And thats the limit since each card has to have its own memory....imagine if they could have 1gig and both cards could use it(like how core2 has a shared cache. now that would be a dual core gpu and not two cards slapped together). As it stands now, high res and high AA do effect the small 512megabyte buffer.

Just my 2 cents

@Maz, because they are counting the GPUs like you are.  Two 3870x2 in CF would be four 3870 GPUs, hence "quadfire".  As I said, its all how your count.  PCBs, you have two.  GPUs, you have four.
 
@nuke, actually you might have just proved my point.  
 

 
The x2 only has the three standard outputs that all video cards have.  The 9800GX2 however has four, two for each card.  Go read some reviews and figure out whats underneath those heatsinks.  Obviously you guys aren't understanding/believing me.
 
3870x2 http://www.newegg.com/Product/Prod [...] 6814121228
9800GX2 http://www.newegg.com/Product/Prod [...] 6814500026

I think i got what u are saying  
u are saying 9800GX2 has 2 PCB so its actually 2 cards
HD 3870X2 has one PCB so its one card
right ?
 
and the reason anandtech says its 2 HD 3870 is..........?
 

so ?

I only see 3 outputs 2 dvi and one hdmi, but for all i know the DVI and HDMI are linked(like on many dvi/hdmi onboard cards)....

Either way its not 4 so

I know EXACTLY whats under the heatsinks....2 cards on one pcb(ATI) and 2 cards on 2 pcbs. Either way the lack of Svid for SD tv users on the 9800GX2 SUCKS.....

Look X2 with 4 video outputs
http://www.techspot.com/review/86- [...] d-3870-x2/

The reason Anand says it is two is because there are two GPUs, and you are already using one of the CF links.  When you say card, I am old school and picture the PCB.  Its a bit like saying the C2Ds is two CPUs.  It's two cores, on one PCB.  I don't see how this is any different.
 
@nuke, I was referring to the only pic of the 9800GX2 that I've seen.  Looking back at it, it is the 2 DVI ports, one DMI, and the fourth that I saw is probably an optical out for audio.  Tell me what you think it is.
 
http://www.tomshardware.com/2008/0 [...] e_9800gx2/

ok thanks mate

Wanted to add that Nvidia's new 750a does support SLI for AMD systems.