well after the launch of the 4890 i was seeing people getting mad after the card ,claiming it one of the best architectured gpu by ati etc ,i dont disagree but im am not at all impressed with it also . well let me tell you the changes made by ati in 4870 to bring down the so hyped 4890.
hmm The stream processors remains same (800) .
# of transistors For ATI Radeon HD 4870 956 million
# of transistors For ATI Radeon HD 4890 959 million//Note this
Clock speed(core): 750 MHz vs 850
Memory Clock :3600 MHz (effective)vs3900 MHz (effective)
Net computational power :1200 GigaFLOPS vs 1360 GigaFLOPS
Bandwidth:115 GB/s 125 GB/s
so as i asked you ,underline the a little increase in transistor count ,that gave the 4890 a room to attain higher clocks, so all we see is a increase in core and memory clock leading to increase in bandwidth and net Floating operations per second(FLOPS),to confirm this try over clocking the 4870 yourself and see that if you are getting the same figures of bandwidth ,texture and pixel fill rate and FLOPS or not .. ooops you dont have a 4870 ,dont you 
,jus visit Gpureview and try it out yourself ,you would be just amazed to see the exact same parameters of a 4890 from a 4870 with same memory and core clock ,ive kept both the cards to be compared for you ,check out here
so if you want the performance of a stock 4890 ,you can try out overclocking your 4870 with an after market cooler to 4890 clocks ,it should put down really close figures to that of a default 4890
