NAT ADSL router with port mapping and dialup modem

Hi,
I was hoping someone could point me to a reliable and low cost ADSL router with "port mapping" functionality (rather than just port "forwarding" I'd like a port on the LAN to map to a different port on the WAN)
Also external access rules so only certain IP's can access ports if I so wish.
Lastly it needs a dial-up modem for backup purposes.

The only thing that fits the bill that I have found is IPCop / Smoothwall and while I really like IPCop, the problem is that it takes a whole PC to run it, which gives space and power issues compared to standard routers.

I'd like to build one using micro atx architecture, but I'd like to know if anyone has gone this route. Also can anyone recommend current hardware that would play with little or no configuration? (IPcop is pretty good with hardware when it comes to NIC's, but I've had no experience of it with PCI ADSL cards or PCI/external modems)

Anyway, if anyone has anything to offer on this subject, I'd like to hear from them as my business partner got all excited on a website with his credit card a couple of months ago at the idea of being able to do this and ordered the microatx PC It'd be a shame to leave the parts lying around for another couple of months!
(it's late and i'm tired, I hope this makes sense!)