AMD has this thing called Eyefinity, and it allows the user to run 3 monitors on a single graphics card. One of those ports must be a Display Port though. Generally, monitors do not come with display ports on them, so one will need a Display Port adapter, which converts it to a DVI signal.
Now, if you are buying a Dell, chances are that it will not come with a PSU (power supply) of over 300W-350W. In that case, one of the best cards that will actually run on that PSU is around a 5670/6670 (they are the same, just rebadged)
Here is an example:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1366279&CatId=7005
It must have that "Eyefinity" compatibility to be able to run 3 monitors and have a landscaped display.
If gaming is on your list, this may be an alright card, just don't expect Battlefield 3 or anything. If you want gaming, a 6870 with a better PSU would do nicely within a general budget.
In response to DM186, you can run 3 displays with 2 SLI'ed cards (I believe) but you would need a large power supply, a bigger case (probably wont fit in a Dell case) and another motherboard, as the board in the Dell computer is most likely M-ATX form factor and will contain only one PCI-E x16 slot for a graphics card.