Remote Desktop

I'm trying to help a very small business access a computer across the net. I
can use Remote Desktop within the LAN. It's a peer-to-peer XP Pro network on
a Linksys BEFSR11 router and a cable modem. The inexpensive one you can find
in most retail shops. They don't have their own IP range. They can get a
static IP from their ISP. (For now for experimenting I can just get the
dynamic IP the router got assigne dby the ISP.) I can make a system in house
exposed to the internet or do some other method? But how do I find that
particular system acorss the net? Since they don't own their own IP range if
I just use one of the addresses in the router's range I'll probably end up
being redundant with other systems out there.

Thanks,
Tom