Prior to mainstream adoption of the PCIe bus, configurations of more
than two monitors were either achieved with an AGP card with dual
video outputs or by using an AGP graphics adapter as the primary
device and a conventional PCI graphics adapter as a secondary device.
However, given the bandwidth limitations of the older PCI bus[citation
needed], such setups were not common, and maximum overall graphics
performance could be obtained only by using specialty solutions such
as the Matrox G450, which features four outputs from one graphics
adapter. Now that computers with two or more PCIe interfaces are
popular, middle- and high-end computers are no longer limited to two
monitors driven by a single main graphics adapter. If a dual PCIe
interface is not available or is otherwise occupied, a standard PCI
graphics card can still be used to provide additional video outputs,
albeit with performance trade-offs. Specialized
application environments such as CAD, day trading of corporate stocks,
and software development are increasingly using six or more monitors
on one production system.

A quad-monitor Digital Audio Workstation Additional monitors can also
be connected to PCs via a USB connection such as DisplayLink.

Seems that it doesn't matter much if it's connected to a PCI Express slot. If it's just a normal PCI (or maybe an AGP) it's recommended to use 2 seperate graphic cards.

Source: http://en.wikipedia.org/wiki/Multi-monitor#More_than_two_monitors

Also, here's a PCI Express graphic card that support 4 monitors. Don't know how Ubuntu compatibility is though.

http://www.nextag.com/VisionTek-Radeon-HD5570-PCIe-827596030/prices-html