Azgard, you have more than one option available. (as mentioned)
1. Build a smoothwall router (Cheap & Great for future expansion)
2. Purchase a cheap Wifi router (Little more expensive but simpler)
3. Get ICS working. (Free, little room for expansion, less control)
1.Firstly the 10mbps card will NOT be an issue if your only planning to share an internet connection to a few machines.
You need to download the iso from
www.smoothwall.org. Burn it to a CD and boot it in your old machine. From there you pretty much follow instructions. Very simple and there is enough documentation available if you encounter problems along the way.
You will ofcourse need a way to link this to your existing machines, IE a switch, if you dont have one lieing around you can easily pick one up around the R200 mark. If you have an old hub lieing around that will work aswell.
Depending on how you have chosen to configure smoothwall you should be able to allow your machines to obtain network information automatically from the DHCP on you new linux box.
This also makes it easy to expand if you add any new machines in the future and should be well worth the time spent setting it up.
2.Alternatively you could purchase a WIFI router for under R500, I think you can pick up the Mecer units for R400 these days. Which would save you the time and effort of building you own box.
3.For ICS to work the host machine (with iburst connected) must have the following IP address. 192.168.0.1, you should let the client machine obtain its address automatically or configure it to 192.168.0.2 (the last digit can be anything).
You also need to make sure that the Iburst connection is allowed to be shared, this can be done in the properties of the network connection created by the USB device.