Normall, you'll want to set things up such that eth0 is connected to the WAN side and eth1 to the Client (LAN) side. In the case of computer A, you might want to change the configuration such that eth0 is attached to your wireless access point, and eth1 then connects to computer B.
As such, let me see if I have this right:
DHCP -> 192.168.0.0/24
WAP (gets address via D-Link Router DHCP)
[eth0] = 192.168.0.104 (static)
: since Computer A is within the subnet, it should have
: a default gateway of 192.168.0.1 genmask = 255.0.0.0 or 0.0.0.0
[eth1] = 192.168.1.1 (static)
[eth0] = 192.168.1.100 (static)
: since Computer B is provided a routed connection via Computer A,
: its default gateway wants to read: 192.168.1.1 genmask 255.0.0.0 or 0.0.0.0
Also, on ALL machines, make sure you have established your resolv.conf file with one (preferably 2) DNS server entries (for your ISP).
Try, on Computer A, pinging out to a www address. You can also try the switch -I [interface] as in:
ping -c 2 www.ascap.com
(The -c 2 will ping twice, instead of continuously.)
If this works, try it without the -I switch. If this works, your routing on Computer A is good.
Now try, from Computer B, to ping each interface, starting with 192.168.1.100, then 192.168.1.1, then 192.168.0.104, then the WAP IP address and finally your router. If you get back to the WAP or router, your iptables script is working.
This will test things out sans internet.
Then try pinging a www site from Computer B. If no joy, delete the default route and replace it with:
route add default gw 192.168.1.1 netmask 255.255.255.0
This is the sum total of my knowledge