RV740 = 40nm, replacement for 4830 and most likely 4850 (but will probably be slightly slower than 4850 with AA turned on because it only has 8 ROPs)
128bit DDR5 = same memory bandwidth as 4850, cheaper PCB, $100 USD MSRP

RV790 = 55nm, replacement for 4870, same 256bit DDR5 but probably with faster chips, should be using the same 4870 PCB but with an extra PWM phase. Rumors about 960SP and 48TMU are probably true, so expect up to 20% boost in FPS for shader intensive games. Pricing unknown, probably still cheaper than the 55nm GTX260.