A quad core cpu isn't required. Even an Atom will push that kind of bandwidth. The number of users doesn't matter so much as the throughput (in both bandwidth and PPS) when considering the CPU. You may need more RAM (about 2GB will suffice) though.
More importantly, what packages/ features do you intend to run? Packages like Snort, Squid and HAVP will require more processing power and much more ram.
If you need VPN on that link, then you should look into the newer Core i processors.