I can't see them offering any new CPUs, as I really don't think there are any options available from Intel to work with the current iMac CPU socket in use today. Maybe there are lower power versions of the same CPUs available, but I don't think there are going to be much gains in terms of CPU speed.
Since the last update, nVidia has released a new GPU line (200M line), which features significantly faster and much much more powerful GPUs.
While the possible August/September (new) iMacs might not gain much of anything in terms of CPU speeds, it is very likely for them receive massive upgrades to their graphical capabilities.
The 9400M used today could in theory be replaced with the 210M, as that is the current generation's lowest base GPU from nVidia. As you can see, the 210M is a major step above the current 9400M offerings from Apple today. I'd go as far to say that they would match the nVidia GT 130 in terms of specs and performance.