Yep - it is likely that radeonHD will get merged with radeon - much of why it exists is no longer relevant.
Still - the features page
lists 3D element support as "mostly" for R600. Though the DRI wiki just mentions "many shaders missing". RadeonHD features reflect a more Cathedral-style development: stuff is either done or not started.
Both radeon and radeonhd have an overlapping feature set, and both should support all r500-r700 cards. If either does not work for you, try the other one.
I dunno - I just have a hard time with the kernel preempting mode affecting the loading of modules in this way... but apparently it does. This prompts me to look in different directions JIC.
I also suspect that recompiling the kernel will be easier than downgrading - and I expect there will be an official kernel update for -current soon.
Normally I don't see trouble with the fglrx packages, just the ati installer and that unusual. But: I have had three of these just this week, all newly unsupported, and for some reason each one has ended up needing large chunks of X removed and installed before the free drivers could work. (None this card, and none slackware.) Where I have been able to preempt the, usually blind, proprietary package install, there has been no problem: it just goes.