Without taking time to verify it, my suspicion is that AMD had dropped driver support for the HD 2000s, HD 3000s, and all but the top three or so of HD 4000s, before the game was even in its actual Beta period. Anyway, as pointed out, making comparisons over the major changes in architectures between the HD 2000s and the HD 4000s is an unclear area. It's probably best to go either upward or downward first, putting the HD 2600 up against an HD 3650 initially, and then that against an HD 4670, then after taking note of the differences along the way, finally compare the HD 4670 to the HD 4870,
. . presuming there is no existing set of benchmarks from another Frostbite game in which all four have been tested . .