>>>>> I have many slots available. <<<<<
You need to be careful with that. Having many available, empty slots is one thing. Are they all of the same kind? For example, many motherboards have a mix of the old PCI, or the older AGP, along with the new (current) PCI Express buses. Mostly for providing an upgrade path for the older peripheral hardware. Mixing graphic cards with different bus types (e.g. one PCI card, one PCIe X16 card) should be avoided if you don't want potential headaches.
Assuming if you have 2 or more PCIe X16 slots, then the solution would be easy. Buy 2 PCIe X16 dual graphics card. The best is the identical model, identical make. If not, at least identical make, different models - which usually works out okay. But with different models, different makes, again that is asking for trouble. Compatibility and driver conflicts can be big issues.
As to "what's best"... it really depends on what your goal is. If you want a no-frills graphics solution for trading, I have been using the low-end EVGA 8400 GS cards (dual monitor each). You can have one for less than USD $30. Nothing fancy. They work. And fairly reliable. (I have about 8 of them and haven't had any failed on me yet, for about 2 to 3 years now.) Though they won't earn you a bragging right for playing computer games with fastest, most photo-realistic animation. For plotting price charts only and 256 basic colors, they are more than fine.