I haven't tried it, but I think it would work, at least if you have a WAP that can be configured as a bridge.
Another option that I think would work, if you have WinXP and don't mind leaving one machine on all the time: Get a USB or other wireless card, and connect it to one of the PCs. Have winxp create a bridge between that pcs wireless and ethernet networks. Then, hook both PCs up to a switch.
I just did something very similar, using hpna rather than wireless; see
this thread. Or, if you don't want to wade through all the details, here is the critical part, from the winxp help. In your case, you are trying to mix ethernet and wireless networks. So, you either need a standalone bridge, such as I suggest in my first sentence, or you need to let a PC act as your bridge:
Using a mixed network environment. When planning your home or small office network, you might want to use a combination of Ethernet, wireless, and home phoneline network adapter (HPNA). For example, you might have two computers in adjoining rooms that are connected using Ethernet adapters and a network hub. If you have other computers in other areas of your home or small office, you can connect those computers to the network using a home phoneline network adapter (HPNA) or wireless network adapter.
In this example, there are two computers in adjoining rooms that are connected using Ethernet network adapters, cables, and a hub. One of these computers is running either Windows XP Home Edition or Windows XP Professional. There is a third computer in your child's bedroom, and a fourth computer in the den. The bedroom and den computers each have a home phoneline network adapter (HPNA) installed and are on a network together. To connect the two HPNA computers with the Ethernet computers, install a home phoneline network adapter (HPNA) in the Ethernet computer that is running Windows XP. If your laptop computer has a wireless network adapter, you can join it to the network as well by installing a wireless network adapter in one other computer on the network that is running Windows XP.
In a mixed network, also called a multi-segment network, one or more computers have multiple network adapters. Windows XP Professional and Windows XP Home Edition have network bridging, which allows the computers and devices on each of the network segments to communicate with each other. Without network bridging, the computers connected using Ethernet can only talk to each other, and the same for computers using HPNA or wireless. Network bridging makes each of these network segments transparent and appear as one continuous network.