I fully expect the price/performance of the 275 compared to the 4890 to be very similar to that of the 4870 to 260 (1gig and 216 versions respectively) we see currently.
Based on the sepcs here, and assumign ATI doesn't release something totally surprising in april the 275 shoudl edge out the 4890, with the 4890 coming in a few dollars cheeper. I dont see the landscape changing particularly much.. just a couple revision cards coming out to replace what we are seeing now in the same price segment.