Not sure what you mean a quad head, since it's a business desktop I would recommend any new Quadro or FirePro cards, not sure on prices but these cards are the same as Geforce excpet have slightly more muscle an reliability.
Some of the desktop quad outputs can be a little flaky from what I know. You are best off getting a workstation card. These can be expensive. Below are some options for you. 1 being a single 4x DVI card. However, I believe the 755 has a PCI-E x16 slot and also a x1 slot, They do make workstation cards that are PCI-E x1 and work quite well. You could install (2) dual output cards. See below.
Most of the 5870 6-output cards are coming with all the necessary adapters you'll need, but you'll probably need to pick up another adapter or two, yes, if you don't have HDMI or DP on your monitors. That said, your total cost should be no more than $620-$650 with the adapters and you'll be able to go to 6 monitors should you want in the future.
If you only have a single PCIe slot available, the Quadro 420 is a good option for the price. The 290's only do dual. If you never plan on doing anything DirectX related (gaming or autocad) that's probably your best option for the price as it comes with a 4 DVI adapter. I'd still lean towards the 5870, but for just 2D it's probably overkill.
I'm always a little hesitant buying video cards from eBay, though that's hypocritical because I've sold quite a few there. heh-heh. The 4850x2 is a good option, too, if you're okay with used.