[CentOS] Build a Firewall (Can I learn to do this...)

Hi All,

I have a home business circuit and I am gearing up to host my business  
affairs in my place. I have Comcast and 13 static IP's.

I have an extra PIII 1U, 2 9gb SCSI, 1gb RAMm dual NICS.

So I am wanting to build a firewall to front end my traffic. Assign  
one of my statics to it and have Comcast statically route my traffic  
to this IP. Then when traffic comes have it decide if it is allowed or  
not and if allowed pass it to the right server based upon the rules.

I used to work with PIX 525's so I have knowledge, I just dont quite  
know how to do this with CentOS and such.

Can anyone offer advice?

Best,
-Jason