Radeon HD 2600
Mainstream products with 120 stream processors, GDDR4 support and AVIVO HD with UVD, an 128-bit memory ring bus width  and 4-phase digital PWM. Neither the GDDR3 nor GDDR4 reference PCIe designs require additional power connectors, however AGP XT variants will require additional power through either molex or 6pin power connectors. Official claims are that the Radeon HD 2600 series consumes as little as 45 W of power.
Radeon HD 2400
Low-end products with 40 stream processors with AVIVO HD and UVD, not implemented with a ring bus memory interface, and a 64-bit memory bus width . The official PCB design implements only a passive cooling heatsink instead of a fan, and official claims of power consumption are as little as 35 W. The RV610 core used in Radeon HD 2400 series has 16 KB unified vertex/texture cache from dedicated vertex cache and L1/L2 texture cache.
different generation of graphic cards are hard to compare, not only they are different in clock, tech, etc, newer cards also support newer standard (such as HD2x00 supports DX 10.0). so generally, newer generation of cards should be more future-proof.Does anyone know if the 2400 is better or worse than the 1600 found in previous iMacs? I cannot find any benchmarks to compare the two. Common sense would suggest that the 2400 is better, but then again, common sense is not always right.
I assume (hopefully correctly) that the 2600 is unquestionably superior to the 1600.
the result in the link is 1650xt, not 1600, 1650 is about 10%~35% faster than 1600only number i can find is here: