Does it
really need to be
PCI-e only and is there
really no other way to add in another card?
that being said, here are a few options:
http://www.newegg.com/Product/...x?Item=N82E16814195086
actual workstation card consumes very little power ( 16 Watt (2D), 26 Watt (3D) ) and comes with some decent looking software conducive to multi-monitor workstations
http://www.provantage.com/jato...x309-quad~7JAT901U.htm
the poor man's workstation card, can power 4 monitors via dual Radeon HD 3450s. Not sure about driver support (for instance, I'm not quite sure how it works or if you could use drivers directly from AMD, however Jaton seems to limit users to 32bit OSes with the driver packages they offer), this one might be a bit riskier compared to the FirePro 2450
Biggset con about those cards is that they're all single link DVI meaning you'll be limited to 1920x1200 LCD panels.
If you need support for 2560x1600 monitors then you're going to have to hunt down one of the discontinued 3800 X2 or 4800 X2 cards that came specialized with four dual link DVI ports.
Although it would be ultimately be cheaper to go for a 5700 or 5800 and then pay for one of those expensive display port adapters.
Anyway we look at it, if you're really limited to
one PCI-e card you're going to be spending at least ~$230.