The Alix should be good for ~85Mbps routing/NAT so you may be OK. Not much headroom though.
An Atom based box will be good for around 500Mbps but even that is quite a step up in power consumption compared to the Alix. 5W vs 25W or there abouts.
Use a VM as you have suggested if you need to.
That's true, but only in one direction. If you're downloading at 80Mbit/s you won't be seeing 20Mbit/s back in the other direction.
The next step up would be an Atom box, mine have been able to push about 300Mbit/s TOTAL (upstream + downstream), though an i3 has about the same power consumption (if done right) and is a LOT faster (as in, wire-speed on Gig-E with a high-end i3).