people have pushed that on 500 Mhz Geode boards like Alix and Soekris boxes (of course this depends if you are using vpn)
check out some stats at PFSense.org
Agreed. That'll be no issue at all. Is this going to be a datacenter (backbone or edge) router? I'm fully Pro-FreeBSD but if you're going to use this as a datacenter router then I'd recommend OpenBSD. It has a little tighter integration with CARP/pf/etc.
Haven't seen a common PPS issue in the last couple years. Hardware has come a long way. It has never really been a limitation of the OS. Always been a limitation of what the hardware can process. (Especially seen with complex filtering)
I don't want to quote an exact number because I haven't done any tests on that hardware. Although, I'd expect it to handle a couple hundred thousand without issue. In our datacenter in Memphis we're using Core2Duo's on the edge. We take some HEAVY attacks and the systems will never chug. We fill up gig pipes before we see the systems take a hit.
Use a DDoS traffic/load generator and let us know your test results. I haven't seen results published in real world tests in a long time.