If nVidia drivers don't work then you might have to look around for custom PhysX drivers. Good luck though, you're not nuts... it's more common than you'd think. I've seen a guy who managed to get an HD 5970 and GTX 260 working together well.
I'm running almost the exact setup now. I have an i7 920 @ 3.3-3.7 (usually at 3.3, as I rarely need more) with 5870's in CF, with an old 9800gt I had in an old machine. It works great on a Antec 850 PSU. However, I don't push my OC as high as you are, on the CPU, but I doubt an extra .3 GHz will make that much of a difference.
It works great on the games I've used it with. I don't use physX very often, but I keep putting it back in, because it makes running on an extended desktop headache free.
You'll be A OK. I'm running a GT240 for PhysX and it works great so far in Metro 2033 and Batman AA (only played the demo to test it so far). As far as I know the 9800GT is slightly better than the GT240, and Batman so far stresses it the most to about 50% usage max. Metro 2033 doesn't stress it more than about 15% normally tho it's been as high as 25%. Also, it's in a 4x PCIe slot...
Also your PSU will be fine I have a corsair 750TX and running crossfire 5850s plus the PhysX card
I tried the beta drivers with no luck. They might have already "fixed" the compatibility thing. I'd say just do the GenL thing... you install 197.45 (newest certified) drivers and PhysX (use the Metro install as it's the latest) then run GenL V1.03. Works fine.