Don't bother PvPing at 19. Casters are useless and are 2 shotted.

2 Wows is pushing it - really depends on the video card and if you have a second x8 or x16 pci-e slot. Also what you consider to be playable. For PvP, 10 fps and load time lag is going to get you killed.

3..... is not really going to be playable.

If you bought a quad core, 4 gigs of memory and 2x x16 1x x8 mobo (most do not do this - thats a $300 mobo) with 3x mid to high end video cards.... and 3 monitors then maybe. But thats a 4-5k box, vs $500 per box for 3.

Personally, I would buy 3 boxes. Try it with some trials but I think you will be disappointed.