Firewall Performance

Hi all, I need to know how much a firewall based an ipchains (no extra software) decreases system performance. I have a server running Linux with two subnets, 1 with all the workstations (currently all MS OS) are connected to, 2 the old 10base2 network with the printer and the PC running NetBSD connected to the Internet. This router has firewalling functionality installed, but if there is not too much performance loss, I thought it was a good Idea to have packet filtering and port blocking active on the server too. Any suggestions and recommendations? Thanks in advance mike