I'd say go fpr the HD 6950, 2 GB. Its a very powerful card and runs about $250 after rebates. This will save the hassles/driver issues of a crossfire setup. The average computer user won't see a lot of benefit from a multi-card setup.
Not sure what you're after with the onboard graphics, there's really no need to pay for onboard video when you are certain to be using an add in card. It will not benefit you in any way, unless your goal is to retire it in a couple of years as an HTPC!
So I would look at a 870 chipset motherboard if that is what you're asking about. They run under $100 US and can even handle crossfire at 8X/8X speed.