As of right now, neither is a great option, mainly because of the new generation from ATI. It's already proven to be AT LEAST 30% better, and Nvidia is taking a little longer, so they might have the upper hand, seeing as they can base a little bit of their 300 series on the HD5k series.
But as for it in a vague form. The $600 will be sometimes better, sometimes it's worse, it's really like a rollercoaster. The $300 will give you PCIe16 SLI, if you buy the correct motherboard, it gives a small performance boost over PCIe8 SLI with the single card, a bonus would be that if the game doesn't support multi cards, you can turn off SLI, so you don't lose performance, it's been proven by multiple people, just look it up. All, in all, if you want the cheapest way for performance, the single card is better since you don't need to buy an expensive motherboard, if you want the safer way, and is willing to pay like $20-50 more on the motherboard, go ahead.
In the end, it's really more about your opinion.
