
I'd have to ask you a simple question... why?
Get a decent Wifi/Router (I trust my D-Link) that has a few ports and a firewall. That way all of your computers are behind your tight firewall and if you want to do fancy stuff you can set up port forwarding and virtual services (all is standard on my D-Link wifi router).
By routing your packets through an intermediate machine introduces a bottleneck on that machine, and pretty much puts a limit on you throughput. Realize that the kernel is doing software NAT for you, but a router is faster because it is all hardware.
From a security stand a router and firewall with optional virtual service redirects is the way to go. Plus if your machine goes down you aren't stuck having to re-configure it's replacement all over again. And don't forget you'll not only need to setup your NAT, you'll also need to setup your own firewall with iptables (or ipchains, if it's still around)...
So you can see more benefit to going with the router...
1. Security
2. Performance
3. Scalibility
4. DHCP and Virtual Servers
5. Cost (My D-Link cost me $40 about 2 years ago).
Hope this helps...