I'd get the single ATI HD5850. It has Direct X11 and GDDR5 Memory. Plus you'll have the option of adding another one later, for supreme power!!
Although two 9800GTXs will be better than a single 5850. Make sure your PSU can handle it. And not all games will support SLI, which means since it's SLI, there's a chance that certain games will still run faster on the 5850.