Looks like on the second card, it has a PCI X1 slot.
it's PCI-E dangit! PCI-E and PCI X are completely different things.
The PPU won't make any difference for SLI. If a game still has tons of really big textures, complex static worlds, high poly models, normal maps etc... then dual video cards will still help.
The point of it is, with your graphics card no longer having to do a crapload of mathematics for physics and your CPU as well....it'll be focusing solely on graphics...that means if you have say a 6800GT and a PPU and tried to play far cry with just the 6800GT you'd get for example 40FPS....toss in the PPU to take care of all the physics calculations you get 80FPS....(examples only of course)
The PPU practically makes your card 'more powerful' in a sense with it having to take off quite a bit of load.
SLI will be unnecessary because the current single card solutions will be more than enough to handle the screen rendering with the physics calculations no longer in their hands.
So like I said, in a sense your current card would then become more powerful as it will be able to focus solely on the graphics rendering.
Cards like the 6800GT (mainly overclocked) and the 7800GTX and now the ATIs X1800XT or whatever it is, can already run games maxed out....if their lifetime was a year in terms of maxing out games, then that lifetime would be expanded with the arrival of the PPU which will just make it that much more clear you don't need this 2 cards BS to play games maxed out.
I'm all for the PPU's all though of course it's just one more thing to suck up money, one more thing to suck up power in your case...blah blah.
Water cool and overclock that beotch!
EDIT: Those demos are pretty cool. It'll definitely make a nice impression in the physics world as those are just demos, so imagine when game developers spend years on a game assuming a PPU will be behind physics...should be amazing stuff.