Anybody know about this?
I'm planning to plug the new geforce 9800gx2 PCI-express 2.0 in my old P35 motherboard.
will I loss some performance?

 
Its all relative as we dont know anything about the rest of your system, but i doubt that the slot would be the thing to cause any performance issues.  As long as you have a good enough system to keep up with it you will be fine.
  mactronix

I'm not even sure if the card will work in a non SLI motherboard.  I'm not saying it will, but I haven't seen a single review where they tried it in a P35 motherboard.  Someone please share if you have seen a review with a 9800gx2 with a P35 or other motherboard with just one PCI-e 16x slot

I think it will not run slower, an least not more than 1-2% slower.  I saw some benchmarks testing this for some other cards, and the results were virtually the same for pcie-1 vs 2.

San Pedro, that was one of the main improvements of the new GX2 compared to the old 7series GX2.   It can be used in any motherboard with or without SLI support.
 
EDIT:   In answer to the OPs question, it may have some effect on total performence, but you will never notice it.

Supposedly it does blackened, but the only thing I heard about it was a pre-release Chinese review saying they could only get it to work on an SLI motherboard, and I haven't seen any other review contradict it yet.  I don't doubt that it does, but I would want to make sure before I purchased a $600 graphics card.

I wish I had $600 laying around to try it out for you..  

Thanks bros!!
Just read from anandtech review:
"the GPUs are connected via PCIe 1.0 lanes in spite of the fact that the GPUs support PCIe 2.0. This is likely another case where cost benefit analysis lead the way and upgrading to PCIe 2.0 didn't offer any real benefit."
 
So I think it's more likely to be compatible to the pcie 1.0

When you get it let us know how it works in your motherboard.  I'm sure others who have P35 mobos and the cash to drop on one of those cards would be interested.

It wont flood a PCIe 1 lane at 16x. Itll be awhile before that happens, next year moren likely

Bro SanPedro, this website tested GX2 with X38 mobo, so it really doesn't need SLI mobo.
Anyway as you wish, I will inform as soon as I get the card, which is as soon as tomorrow, the selections are: albatron $660, winfast $760, gigabyte $740  
i might choose gigabyte, good match to my gigabyte mobo