I'll leave the ultimate selection of VPN provider to you (too many to pick from). But as far as configuring the VPN client, there's plenty of documentation over at dd-wrt.
http://www.dd-wrt.com/wiki/index.php/Static_PPTP_VPN_Client
The one thing I'd strongly recommend is using a separate dd-wrt router for the VPN client by configuring it's WAN w/ PPTP. It's so much easier and adds a lot of flexibility. Anyone or anything that needs the VPN simply connects to the VPN router, but otherwise, uses the primary router. No messing w/ the firewall and iptables.
[primary router]<-- wire -->(wan - pptp)[vpn router]
To that end, you might want to visit StrongVPN.com first and examine their numerous tutorials that describe the process in depth, for many types of VPN clients, including dd-wrt (and they demonstrate PPTP over the WAN, my recommendation).
http://www.strongvpn.com/setup_dd-wrt_pptp.shtml