Always go for a single, more powerful GPU. You'll typically run quieter, faster and cooler. SLI works great for some games, but overall you'll get very uneven performance.
Before upgrading check to make sure you've go the PSU to power it and a processor that won't bottleneck it.
I have a amd 6300 at the moment and my power supply is only 600 I know I have to upgrade that this is my first build so I'm still learning some stuff.
Always use a single card solution over SLI if you have the choice. 600 watt PSU should be plenty for one card, if its a quality PSU. Some of the more sketchy manufacturers play with the numbers and inflate the power ratings so they can charge more.