Sum up: you can upgrade a PE800 server into a desktop by adding a sound card and a video card, but not possible to use PCI-Ex16. Upgrade to PE830 motherboard: still only PCI-Ex4.
From an owner of a converted Dell Poweredge aPE800 server, not exactly true.
In fact many computers with two PCI-E x16 slots actually only have 8 lanes wired to one of them.
Similarly a PCI-E x16 full sized card will work plugged into a PCI-x1 slot. The power to the card is through the first small section, and the data through the others. One section or 8 or 16 connected is the difference.
The PE800 has plenty of clearance above the card, and even has power connectors for old IDE devices to use for a video card instead.
Using a standard card (HD2600 XT) with no power plug and dual monitors running two different videos causes a bit of a lag through a PCI-Ex1 slot.
How to connect: Do NOT cut up the slot, because the card has to be elevated above the heatsink anyways. Buy a cheap slot adapter.
PE800 limitations - I had no luck using TWO PCI-E video cards in the two PCI-Ex1 slots.
Even the Dell ATI HD7570 1GB (happens to be LP) is ready to go for Windows 7 or Windows 8, just buy that 1 lane to 8 -16 lane adapter.