I need to create several distinct VLANs, and provide a way for traffic to move between them. A "router on a stick" approach seems ideal:

                                Internet
                                   |
                      Router with Trunking Capability ("router on a stick")
                                   *
                                   *  Trunk between router and switch
                                   *
                      Switch with Trunking Capability
                       |      |       |      |      |
                       |      |       |      |      |
                       |    LAN 2     |    LAN 4    |
                       | 10.0.2.0/24  | 10.0.4.0/24 |
                       |              |             |
                     LAN 1          LAN 3         LAN 5
                  10.0.1.0/24    10.0.3.0/24   10.0.5.0/24

We have trunk-capable Layer-2 switches. The question is what to use as the router on a stick. My choices seem to be:

1) Use an existing Cisco 5505 ASA firewall. It appears the ASA can do the routing, but it's a 100Mbps device, and so seems sub-optimal at best 2) Buy a router. This seems overkill. 3) Buy a Layer-3 switch. Also seems overkill. 4) Use an existing Linux Box as a router 5) Use a new Linux box as a router' 6) Something I'm not thinking of

I think either (4) or (5) is my best option, but I'm not sure how to choose between them. I expect the amount of traffic that has to cross the VLANs to be somewhat small, but bursty. How much load does routing add to a CentOS machine?