Load balancing basing on load estimation

[Artjom V. Gora]

Hello everyone,
I'm looking for something like iptables' RATEEST+CONNMARK feature in PF. That is I need a feature which allows to perform load balancing between two or more net channels 
basing on the current load estimation of each, thus it should assigns new connections to line that have more free bandwidth.
Initially I need to achieve following scenario. Say I have web server with 2 nics attached to different routers. If nothing happens it just works as if single nic is 
active. But sometimes traffic burstiness occurs say due to movie streaming or so, thus we quickly run out of bandwidth on 1st nic. If so happens all the "extra" traffic 
should be sent to the 2nd nic, in other words every new connection should be assigned to the 2nd nic only until 1st one normal operation resumes.
Is it possible to implement such scenario with PF? Should I try some other tools for this and which if any?
Any help is greatly appreciated.