Hd5870 & 8800gtx

[dlder]

Hello everybody!

I'm a little bit confused as I posted this message just a few minutes ago... but as it is not displayed I'll ask again

Let me do a little explaining beforehand:
I've got these two video cards working on my Win7x64 machine and confirmed that with several PhysX-Benchmarks & -Games.
At last, I tried the Cryostasis-Benchmark-Demo with everything maxed out (1920x1200, Highest, DX10, PhysX: Hardware) and got whooping 33,1 / 53,9 / 123,5 FPS!

So I told myself: I can finally play this (probably^^) awesome game!

But unfortunately I could'n install the game under my current OS. Then I googled some and found a lot of "... doesn't work in x64..."
I couldn't believe that; I hoped that only the installer is broken.
So I installed/patched it on a WinXP machine and copied it onto my main rig.
And behold: the game started! Yeah!

And because the demo ran so well, I cranked everything up and couldn't believe my eyes: ~20 FPS?! (ok, when viewing the wall or something I get way more, but that doesn't count).
Then I opend the first hatch; there's a lot of ice around that brakes and which causes the FPS to drop to single digits...
Then I disabled Physx and at least got around 30 FPS...

So, here are my two questions:
a) Why is Physx working in the demo, but not in the full game? (got the europe/english DVD if that matters)

b) Why is the performance with the HD5870 so bad? (I mean even when Physx is turned off)

Thanks in advance for any help/comments...