Wait. New cards are coming in just mere 2 weeks. They will have better power consumption, better performance, and most importantly, these new GPU's will lower the prices of the current gen GPU, ie 5850.
You probably won't see any 6000 series cards until November, if you can wait that long.
it's usual that when ATI announces new graphic card line, there's always a wait of a few months before even single card on the shelf + the supply always fall short of demand. so if they announce in next 2 weeks I won't expect a constant supply till Q2 2011
I would plan more on buying a high-end 5xxx series card when the 6xxx series comes out. the price of current 5xxx series cards is already being driven down by Nvidia's new options and will only get lower when the 6xxx series comes out.