How does dual graphics card in a computer affect performance?

I'm selecting hardware options for a computer, and you can have either single or dual graphics cards. Is it better to have a single fantastic graphics card or dual medium capacity ones?

How does dual graphics card in a computer affect performance?
In general, a single powerful card outperforms two medium/low-end cards.
Reply:Get the "single fantastic card" so that you'd get more performance but it really depends on which cards there are to choose from....for example, an ati 4870 is stronger than an gtx 260 and its around $ 150 cheaper. And combining two 4870's will be stronger than a 9800gx2 which is also more expensive.
Reply:Single fantastic one.


Two medium capacity should be in SLI mode to do something and I don't think they support SLI mode.
Reply:single capacity card will have to force itself to handle everything


but dual will share the amount of work.