Well the fact that those few cards are using GDDR5 dictates that these cards will launch H2/2008 around same time as GT200.
If these specs hold truth these high end cards could be great(HD3870 vs. HD4870):
103% more shader performance
171% more texture mapping performance
35% mor ROP performance
95% higher memory bandwith
HD4870 X2, HD4870 and HD4850 looks great, but those HD4670 and HD4650..well they'll have hard time against 9600 GT.
256bit memory bandwidth? 384 would have been better.
Remember that when we are talking about how much data can memory channel pass onwards in one second it's not just about how wide that channel is. We have to look for memory clocks also.
These high end models use GDDR5 memory which can reach ridiculously high clock frequencies which will more than compensate narrow interface.
HD3870 32*2250MHz -> 72GB/s
HD4870 32*4400MHz -> 140.8GB/s
8800 Ultra 48*2160MHz -> 103.6GB/s
HD2900 XT GDDR3 64*1650MHz -> 105.6GB/s
HD2900 XT GDDR4 64*2000MHz -> 128.0GB/s
As you can see: HD4870 has more capable memory channel than HD2900 XT even when memory channel is twice as wide in HD2900 XT