I suggested overclocking in his case because the i7 920 is clocked rather low at stock, and that can lead to a CPU bottleneck when gaming with two higher end graphics cards. If you have one of the higher clocked i7 900 series CPUs, you may not need to overclock at all to avoid CPU bottlenecking, though that would depend on which cards you got.
Current graphics cards are capable of supporting more than 2 displays, so you don't necessarily need to go for SLI or Crossfire in order to hook up everything you want to. Starting with the GeForce 600 series, Nvidia cards can run up to 3 displays on a single card. Any AMD card that is a worthwhile upgrade from your 5870 will support at least 4 displays.
The catch is, your third display and up needs to be connected via DisplayPort. If you do go the single card route, make sure your third monitor and your graphics tablet have DisplayPort inputs, or you will have to go out and buy active DisplayPort adapters.