It's called the grpahics card shuffle. If and I mean if, these 6 series are going to offer 30% performance boost and be cheaper then selling your 5850/5870 a week or two before and then upgrading can keep costs down.
I have always tried to do this and even got a decent price for my gtx260 when the 5850's came out.
Especially that we usually know both pricing and performance before the release.