Well the cheaper of the Nvidia cards is supposedly going to be $200 more than the HD5850. I'd only wait if you really need something more powerful(which means your monitors resolution is 2560x1600 basically)
I would wait because even though the Nvidia GPU's are supposed to be much more expensive, I think we might see a drop in ATI prices around the launch time, or shortly after. You may end up with a 5850 still, but if you aren't desperate to have it now it might save you money/get you a better card.
I'd wait since we're already this close to the "release date"(I'll start to believe it when I start to see reviews) worst case scenario the price will be the same and you'll have more choices, and since the 5xxx have been out for so long wouldn't surprise me to see a price cut just to make Nvidia's life difficult.
I am thinking the biggest thing to drive down the current 58XX prices is the March(?) release of the 6 displayport Eyefinity models with 2GB RAM
I don't think the Eyefinity version or 2gb models will cause the regular 1gb versions to lower in price, most likely they will be released and end up getting a huge price premium so there wont be any reason to lower the regular versions.