I have a P35-DQ6 that's almost a year old ,
I think the motherboard has a PCI-E x 16 1.1 interface,
With the dropping prices, I'm offered a GTX280 at a good price and I'm thinking of buying it.
Back in the 8800 days , the performance difference was in the range of 1-3% (unnoticeable, if not non-existent and caused by other factors), but now as the cards have progressed
will i see significant difference between both interfaces?

I also have a question about the power, since the PCI-E 1 draws only 75W from the motherboard , as opposed to 150. Is that a problem ?, given of course there's no problem with the psu wattage ...

ok first, i would not get a GTX 280 at all...either get a 4870 or 8800GT SLi. for any of these choices you will have to buy a new mobo as a P35 doesnt support the 8800GT sli, the 4870 would be held back by pci e 1.1 (the GTX 280 would also be held back but 8800GT sli slogs it in almost every game for at least 2 and a half times cheaper!!!!!!)

either way i would suggest gettin a new mobo - P45 for the HD4870 or GTX 280 (if its around half price off wherever ur getting it from LOL) and an evga 750i FTW mobo for the 8800GT sli rig.

You don't need a new mobo, just get the card you want, +1 for 4870, and pop it in and connect the 2x 6pin power connectors.
The pcie bus won't hold it back much, ~5% at most

^-1.

+1 choose whatever card you want, although the 4870 is at a better price point.

get GTX280 the best card on the planet right now. play crysis to the max, ATI cants handle the heavy cryengine

concrum. look at this and then tell me the GTX 280 is better than dual 8800GT at crysis.
http://www.anandtech.com/video/sho [...] =3341&p=13

and then justify the 2.5 to 3 times price increase from 8800GT's to GTX 280.

I have been thinking, a similar question, ATI 4870, , GTX 260, both have come down to the same price at LCS or even a couple of 9800 GTX+ SLI (although this would be some more money)

In answer to your specific questions...

You will see little to no drop in performance with your PCIe 1.1 vs the newer PCIe 2.0.
Toms tested the scaling of PCIe 2.0 a while back on the 9800GX2.
As PCIe 2.0 @ 8x offers the same bandwidth as PCIE 1.1 @ 16x and the 9800GX2 needs a little more bandwidth than a GTX 280, you can get a good impression of the performance hit.
You can find the review Here.

As long as your PSU is up to the task, you will have no problems supplying power to the GTX 280 in a 1.1 slot. It just works within the supplied power envelope.

You could consider the 9800GT's.

yeah but 9800GT's arent released yet for system builders are they?
Only to Dell and them arent they...

9800GT supposed to be faster than 4870 in every games, especially Crysis.

No.
No it is not.
Will you stop posting on this site already?

I wouldn't worry too much. A Pcie x16 slot (1.1) is enough for a GTX 280.

Selling a graphics card that would not work properly in a PCIe x16 (1.1) slot due to lack of power or bandwidth would have been a very stupid choice by either GPU maker since the majority of computers still have 1.1 pcie slots.

It might take a slight performance hit - but you shouldn't see it like that. It's better to think that PCIe 2.0 will give you a slight performance boost. A neglectible amount though.