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.