First off, which VPN service are you looking to obtain? Most services -- BTGuard, ipvanish, et al, are geared for PPTP
(avoid like the plague due to weak encryption) or L2TP VPN, but I've found VERY LITTLE support for non-DDWRT /
Tomato / et al configurations for these services. IIRC, DDWRT, Tomato, et al will support remote-access VPNs, so for
minimal hassle, I'd check into wrt320n's compatibility with being loaded with alternate firmware and go from there.
For security, as I said, avoid a PPTP setup -- see »www.schneier.com/paper-pptpv2.html
for a full explanation.
AES-128 / SHA1 is considered a baseline... thought AES-256 / SHA2+ is more ideal. To hit 50Mbps with AES-256 / SHA2,
scale your hardware accordingly. Generally speaking, home routers' CPUs are scaled for cost, NOT high encryption
For remote access solutions, there's appliance-based and DIY -- key things is your budget, and level of comfort in DIY.
Also, keep in mind that you'll need a compatible remote-access client, of which appliance solutions typically charge
you on purchase of the device, or on a "per-seat / device-installed-on" basis. There's a "free" VPN client Shrewsoft,
but I can't claim much personal experience with it -- YMMV with it.
Alternatively, there's SSL-VPNs -- of which I THINK OpenVPN supports, but again, keep in mind of your budget and
level of DIY comfort.