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.
I can run a HIS X1950XT IceQ Turbo 512mb in a PCI-Ex1 slot with dual monitors no problem at all.
It requires a separate power plug of course. However, running 3DMark06 results in scores much lower than a PCI-Ex16 setup.
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 - NEVER USE THE PCI SLOT because that slows down other areas of the computer to PCI slot speed! These servers do not have independent slots.
For example, the HD audio part of the PCI-E video card shares an IRQ with USB 2, USB4, the Firewire port on my sound card in 1st PCI-X slot, and the embedded Broadcom NIC.
SUM UP - a PCI card in the wrong slot can seriously slow down a PE800. Install a PCI-E card first. PCI cards work fine in PCI-X slots.