The Lite model is $99.99 and has higher firewall/routing performance than a Cisco ASA 5520.
I would look at PfSense. It runs on any x86 platform and has lots of features including VPNs, Proxy server, filtering, etc. Honestly just as good as an ASA or a SonicWall would be (or better IMO). PfSense is free to download and again you can run it on any x86 platform.
Usually for my clients I get one of these, a stick of 2 or 4 gigs of RAM, and a 30-60 gig SSD and it runs beautifully. The Atom proc is more than fast enough. If you have an old box you can run it on there too. P4 or better is plenty fast. I have over 600 simultaneous users on a 100 meg fiber connection and it never has more than a few percentage points load and maybe 10-12% memory usage, if that.
The EdgeRouter Lite will not to 1Gbit fine, far from it and hardware acceleration breaks stuff you might want to use to don't rely on those numbers. I'm not sure how efficient FreeBSD is compared to Vyatta which seemed to be very buggy at the time I last looked at it but I get around 250mbit doing iperf and I'd say about 15mbit using OpenVPN. That said, its still a very nice device given its price.
OK, then what about the Zyxel Zywall 110 that Mackintire suggested? Will that handle 1Gbps much better than the EdgeRouter?
I guess you could do that but due to the performance drawbacks and limitations most use either OpenBSD or FreeBSD distros as they are. To be honest I've never heard of someone running pfsense in a "mission critical" setup for large networks, many are using/used OpenBSD though. Not saying that pfsense isn't used but I think its pretty rare. You probably want to look into CARP aswell in general.