Any decent DSL modem should operate in bridge mode.
This will make it operate as a "standard" modem.
If you have the option, install an ethernet card for your RED side (IPCop speak for the WAN/Internet).
This way you don't have to toy with drivers.
I have a DLink-504G running in bridge mode, attached to an IPCop box (1.4.1) via a PCI ethernet card, and it was almost scarey to see just how easy it was to make it go
I beleive DLink used to offer a plain DSL modem (the 300 ?), but I like the idea of having a peice of hardware that can turn into a router in case the IPCop box fails for some reason.
The other option is to run the IPCop box behind the route (doing a double NAT), which can add a layer of security for you, but then you'd have two places to check for logs, and IPCop won't let you check the router via web page (unless you install lynx or something), so it'd be more trouble than its worth imo.