My setup is quite antique actually, but considering all it does is monitor feeds it is still fast enough to cope.
I use a 2GHz Pentium 4 with 384 MHz RAM. The graphics card is a Matrox G200 twin monitor AGP with a Matrox G450 eTV in a PCI slot. All running on XP but it worked too on Windows 2000.
I have one LSE real time feed, one Level 2 feed, one IE standard broker feed and DDE the real time feed in to an Excel spreadsheet. It works hard but I have never been aware of any delays.
You don't need anything special to use multi monitors. You will get there with a little perseverence.