i had no problems adding an ati pci express to one of the proliant ml110g3 servers i have running in western dk, there is not much room.. and i tried several cards.. depending on what you need performance wise, the ati card was good, because the onboard gfx chip just cant push hard enuff to run a 22inch lcd monitor smoothly at that location.. so we got it working.. no funky jumpers.. the bios did not help much .. you can not manually disable on this bios ver. but it detected it fine and actually provides video to both outputs if you need dual screen.
i actually cant remember the model nr. its a smaller ati like 1900xt there are many "short" versions that should fit fine in the hp server.. you cant put a big card in.. so forget nvidias.. as im sure you already know
ill post exact model nr. when i find the data sheet.. its not on this machine..
a small ati should work
good luck m.a.