If you're expecting something close to a 50% reduction in price, you'd most likely have to wait until it becomes superceded by newer hardware. And if history has shown anything, the 8800's will drop in price once the HD2900's fall as well.
Personally, I haven't spent more than $200 on ANY video card upgrade. This includes my 9800 Pro 128MB, X1800XT 256MB, and my current X1950XT 256MB (with this one being the fastest AND cheapest ). I'm confident that when the top end of both companies costs around $200, you should still have excellent performance as long as you don't mind that they will be last gen hardware.