I 2nd vmware. I typically run my testing lab from my main system since it has all the juice. I also run a Hyper-V environment on my Win2K8 box. VMware typically gives you the option to run the systems in their own standalone network, bridged or straight through. VMware workstation will run you about 190 (ouch thought it was cheaper) bucks. If you want something for free you can run with Virtual Box, it also allows for snapshots.
The beauty of running these labs in VMs is certainly the snapshotting ability but also you don't need any additional monitors or keyboards to build them.
But to answer your initial question, if you don't need internet and you just want to use your Linksys as a switch for them. You don't need anything fancy. Just configure them with their own network range using static IPs. For instance if your Linksys network is 192.168.0.0/24 set your lab up to use 172.16.1.0/24. you can configure a gateway but that won't really matter. If traffic is only going between hosts on the 172.16.1.0/24 network, they won't even go to the linksys gateway.
If you did need your linksys to route traffic from those lab systems, well you would need to flash it with DD-WRT or Tomato. Tomato allows you to specific additional routes for internal hosts.
Hope this helps.