Don't hold me to it, but I have a similar router (Linksys Etherfast) setup and I've attempted what you want to do successfully. (To use Apache to put a mini Web site, accessable only by the IP address and not by name, out on the Web. Ditto ftp)
(As I remember it. . . .)
You need to:
(1) Log in to your Router via your browser
(2) Click the orange "Advanced" tab
(3) Click on the "Forwarding" tab
(4) Change one of the "0" port entries to "80" (for http/Web)
(5) Designate the LAN IP address of your Mac on the corresponding "IP Address" line.
(6) Change another of the "0" port entries to "21" (for ftp)
(7) Designate the LAN IP address of your box on the corresponding "IP Address" line.
This assumes:
(1) Your box has a LAN IP on your internal network, one that you've assigned by: (a) disabling the "DHCP Server" option on the Router (go to the DHCP tab to do this) and (b) you've assigned a static LAN IP address (like 192.168.1.100) to your Mac in your TCP/IP (9.x) control panel or (X) System Preferences (Networking) panel.
(2) Your ISP will allow direct hits to your box.
OK--this is what I remember about getting it to work on my box with OS X (I seem to recall).
BUT I *do not* know the security ramifications. You REALLY should find out more about that before you proceed. I've seen posts about Apache scripts that need (I think) to be modified prior to "going live" to provide increased security for your box. I was too concerned with security to keep anything open and available for very long.
The other problem you may encounter is if (when) your provider changes your external IP address--you'll need to tell your friends about the change everytime you have it reassigned for them to "see" your box.
Good luck.
Will