I'm running 2 monitors now using an ATI radeon 9600 Pro and it works 100%.  Up until recently I have had multiple thin client boxes on by desk but having one keyboard and mouse is a big advantage.

I was looking at adding a 3rd monitor and was wondering if there are any known drawbacks?

Performance is not an issue as the PC is reasonably grunty and most of the tasks are Remote Desktop sessions so the local PC is doing little except run Outllook and RDP client.

Current card is AGP so I guess the next one is going to be PCI ...