While your question is somewhat vague as to what you man by 'hosting' (website?)
I would just pick up a 4 port hub and then jack each machine into the hub
then out to the net.
|----------------LAN SIDE---------------------|----WAN SIDE|
[machine a] -> [eth0/nic] -> [4port hub] -> [DSLmodem] -> [net]
[machine b] -> [eth0/nic] ----^

as far as resources go..

Windows only.
if youre hosting a website from 'machine a' (gateway)
in your current configuration and want to share out hard drive resources or file
system(s) from machine b through it all they have to be is in the same
workgroup and each has to have a mappable shared drive. then you can map
a drive from b to a and then utilize the resources on 'machine b.'

linux is different, unix is as well. differing OSs can make this far more
complicated than this needs to be.

Also,

check the specs on the DSL modem. You may need to get a router instead of a hub.

Catnip is right:

When using a hub or switch (instead of a router) for that sort of configuration, you need to check with your Internet service provider first.

If the ISP only allocates 1 IP to your account, uses the MAC address of the connected computer as part of your account/connection validation, or uses any other methods which would disallow concurrent logons from multiple computers, a hub or switch won't do the trick. Many ISP do have limit like this, and the choices in that scenario are either to see if the ISP allows you to purchase additional IP addresses, or to use a router instead of a hub.

A) Broadband routers use NAT (Network Address Translation) to allow multiple computers to share a single connection (without your ISP knowing that you have more than one machine connected). Hubs cannot do this.

B) Broadband routers can be configured (via port-forwarding) to allow computers on the LAN (your internal network) to act as Web, FTP, game, etc. servers to the outside world, while still providing some measures of security for those machines. Hubs cannot do this either.