Does a motherboard with two PCI-e x16 slots mean I can run two standard graphics cards (of the same make, ie 2 x HD 6870's) supporting 2 monitors on each, so a total of 4 monitors? So simply don't turn on crossfire or anything fancy.
Yes, you can.
Most graphics cards will support two active monitors out of the three outputs. There are some single cards that will attach 3/4/more, but are not worth it unless this is a gaming system.
Just attach two monitors to each and you are good to go. Crossfire does not even come in to it. The two cards are installed individually with no cf connection.
Windows will detect 4 monitors, and you can use them all as an extended desktop.
For normal desktop use, like day trading, you do not need expensive gaming cards like the 6870. Cheap $30 cards with the appropriate outputs for you will do.
They do not even need to be the same make if you are using windows 7. Same make is nicer, since they will both use the same copy of the graphics driver.