Physx Supported Nvidia card in pcie x1 slot?

Not really. Only a very old modded driver set allows you to use PhysX with an ATI card and even then, you're not missing much. Only a handful of games use it anyways. Why bother?

The best x1 pcie Nvidia card you can get would be a GeForce GT520 and it could probably provide adequate physx acceleration but whether or not it would actually work in your attempted setup, I cannot say. As for using an 8800GT, that won't work as it's not compatible with the slot.

Just look at a diagram of PCIe slots and it will show you which slots are what and the card specs will tell you what the card is and what it requires. PCIe graphics cards rarely come in x1 because of the bandwidth limitations of that type of slot so you won't find them on shelves for the most part as x1 slots are intended for add-in cards though x1 cards are pretty rare all the same since x4 is the industry standard for stuff like USB controllers and eSATA cards. x8/x16 are what almost every graphics card uses though you can occasionally find an x4 card.

The problem is not the card...it is the slot.
All graphics cards will work fine in an x1 slot (if you overcome the obvious physical problems). But mony motherboards have logical limitations on x1 slots which prevent video cards from working in them.

I used a video card for PhysX processing in the x4 slot on my Asus Maximus IV Extreme without any issues

Unfortunately, motherboard manufacturer's don't list whether or not their slots are compatible....you'll just have to try it out and see