As it appears you need to invest in either an adaptor or gfx card, both talking at least $100 as you say.
Just to throw another idea into the mix, would you perhaps consider selling the monitor for another but with DP input?
You could still use the 5870E6 and of cause no flickering.
What is the highest resolution an active DP to DVI adaptor can support?
So far I've seen people use them up to 2560x1600 (as the TH2Go is capped at 3840x1024 on the 5000 series). I could trade up to a Dell 3008, which would double as a TV for me. But I got a really good deal on this panel ($400 with absolutely nothing wrong with it).