ADSL routers/modems and public IP addresses

Hello,

I'd appreciate any help/information on ADSL and public IP addresses.

What I need to do is the following:
the ADSL router should connect to my ISP but my public static IP
address shouldn't "stay" on the router but should be assigned to the
only ethernet card connected to the router (1 PC), as described below:

LINE---ADSL ROUTER---NIC eth0:LINUX SERVER/PC:eth1
NIC---SWITCH---LAN

I've managed to get decent results in just two cases (let
WAN.WAN.WAN.WAN be the static public IP address assigned to me by my
ISP):

1) with the 3COM 812 ADSL router, enabling "Enable Bridging + Enable
routing", giving it a make-believe LAN IP WAN.WAN.WAN.2, RFC1483
protocol
Eth0 (NIC connected to ADSL router) on Linux server has static public
IP WAN.WAN.WAN.WAN (manually set) and eth1 has private IP
(192.168.1.1).

This way, the router seems to function as a bridge and is
"transparent". The Linux server has the public IP address and is thus
on the Internet frontline. This is intentional because I don't want
the router to do NAT - I can do that on the Linux
firewall/gateway/router for LAN PCs beneath "eth1".
Also, I need the public IP ON the Linux box because some programs
don't work (or are a pain) behind NAT (e.g. Linux FreeSWAN without
NAT-T, Asterisk PBX, PPTPD, etc.).

The 3com router config. works fine even if it's rebooted (settings are
not lost and the Linux server keeps being on the Internet frontline).

2) with Alcatel Speedtouch 510 ADSL router.
The protocol is now PPPOA.
Once the ADSL router is uop and connected I can manually delete the
PPPoA static public IP WAN.WAN.WAN.WAN and add a new entry with the
make-believe IP address WAN.WAN.WAN.2.
Eth0 (NIC connected to ADSL router) on Linux server has static public
IP WAN.WAN.WAN.WAN (manually set) and eth1 has private IP
(192.168.1.1).

The result is the same as in example 1), i.e. the Linux server is on
the Internet frontline BUT if the Speedtouch router reboots (power
shortage, etc.), the pppoa entry with WAN:WAN:WAN:WAN gets reloaded
automatically on the router and that conflicts with the Linux eth0's
IP. To solve this conflict I usually need to enter the ADSL router's
config pages and delete the pppoa interface (WAN.WAN.WAN.WAN).

Personnally I don't think the "WAN.WAN.WAN.2 method" is good. Is
anyone here using it?

I also tried another method that doesn't work:
3) I let the ADSL router have its default pppoa WAN.WAN.WAN.WAN
address and set default NAT server = Linux external NIC private
address (eth0). This way all ports are supposedly redirected to the
Linux box. However, it doesn't seem to work with FreeSWAN without
NAT-T because the eth0 Linux NIC address is still a private IP:
LINE---ADSL Router with PPPoA WAN.WAN.WAN.WAN, private IP
172.26.254.1 and default NAT server = 172.26.254.2---NIC eth0 with
IP 172.26.254.2:LINUX SERVER/PC:eth1 NIC with IP
192.168.1.1---SWITCH---LAN

I haven't tried using a USB ADSL modem because I'm afraid driver
support for Linux is weak, in general. Also, I believe that if I use
this type of modem I should get a ppp0 interface with my ISP-assigned
static public IP address. Am I right? In this case (USB modem), will
someone from "Internet" be able to connect directly to the Linux box
by using the static public address? (in other words will the modem be
"transparent"?)

Note that I've only used PPPoA and RFC1483. I never had the chance to
use an ISP with pppoe.

Hope someone can help me out and give me a few pointers.
(I'm not a network expert)

Thanks in advance