I just bought an XFX 4890 Black edition (1000mhz core) to replace my aging 4870 Crossfire 512MB setup. Do you think I'll notice a big performance hit? What is the difference between the two percentage-wise anyway?
Depends on the game, but the performance, saying you are at a beefy res (1920x1080+) will be certainly less. % wise, no idea. Look up some reviews on the HD4890. Im certain some will show vs 4870 in crossfire.
The 4890 should perform quite a bit better at high res, and it is a faster card.. Not sure what you mean by the clock-for-clock comment, but rest assured that you do have a card that is faster than your 4870 with 512MB of memory; if it was a 1GB 4870, the difference could probably just be a few FPS. Crossfire can be 100% efficient, but it really depends on the game. Typically a good 70% though and seems to be getting better all the time.
I believe it is identical clock for clock as the HD4890 has the same amount of shaders and bandwidth as the HD4870. The 4890 however has 100Mhz higher core clock and memory speeds.
As far as scaling. Maybe a game or two can do 100% but its usually no where near that amount which is why I say go with a more powerful single card. Scaling can range from 0-100%. An average I have found is around 50-60%. Not worth it IMHO. Buy one card for the same price and get 100% out of it 100% of the time. Crossfire/SLI especially with HD5850 or above are a waste of money with a resolution less than 2560x1600.
4890 is 6% faster clock for clock, not sure why.
I ditched my crossfire 4890 setup, too much hassle. Your better off with a singly beastly card OC'd to the gills. I could pull another 75mhz on my OC after getting rid of my 2nd card.
the 4890 is NOT the same as the 4870. The 4890 has more transistors and is packed with other goodies which also allow the higher OC. This is why clock for clock, the 4980 > 4870. However, once you CF the 4870, they will beat out the 4890 in almost every game. To really have a better single card solution, you would have to be looking at the 5870.