CPUs shouldn't be a problem. x86 is x86 for the most part and I don't think I've ever seen an OS that wouldn't run on a newer CPU. The only drawback is that OpenBSD is still using GCC 4.2 for some components, so it won't be able to take advantage of some of the newer processor features.
The man page for
radeon(4) indicates that there is support for Radeon APUs. It looks there were some drm and driver updates completed in March and April according to the cvs list. You'll need to run current to take advantage of that.