Radeon R5xx 3D programming guide released

All X drivers are transitioning to using EXA exclusively; the only reason I know of to stick
with XAA is if your card's EXA support is less stable than the older XAA support.

That is unlikely to be the case when using brand-new drivers for previously unsupported
hardware :-).  I'd just turn on EXA.  (And if things break, then file bugs and maybe drop back
to unaccelerated mode, I guess.)