Re: Best choice for HTPC G210 or GT520 ?
The current non-legacy FreeBSD drivers aren't as well exercised as the Linux ones are (the installed user base is much smaller, etc.), but I'd still expect them to perform about the same. The feature set is very similar, with the main shortcomings (again, relative to Linux) being in the area of power management and interaction with the ACPI subsystem. On many laptop/notebook systems, the FreeBSD drivers are likely to fail where the Linux ones wouldn't. But on most desktop systems, these limitations shouldn't present a problem.
I don't know about audi-over-HDMI on FreeBSD, but HDMI video should work fine, and VDPAU should provide accelerated video decoding support (check the README for a given GPU's VDPAU feature set). The emphasis is on should, however, for the reasons I gave above.