Why would you want a PCI-express wireless network card? As far as I am aware, graphics cards are the only available PCI-express hardware available at the moment and they're not exactly mainstream yet.
Anyway, PCI-express equipped motherboards are backwards compatible with PCI, so any PCI wireless network card will work. Alternatively, get a USB one?
What exactly do you need the router for? How many computers do you wish to connect together? more than two?
Do they all want access the internet?
Broadband?
If yes, you need a wireless gateway, not a router, since a wireless router requires the addition of a cat5 UTP DSL modem, which are expensive and very hard to find.
A gateway provides an all-in-one solution for a small amount more money than a router, but works out cheaper because of the modem thing.
Get an 802.11g (54Mbps) gateway and wireless network cards rather than 802.11b (11Mbps).
You'll need a regular (wired) cat5 UTP network card in one of the PC's to configure the gateway via the console. Some motherboards have this onboard, otherwise the cards are very cheap anyway. Once configured, the gateway will be free-standing and completely self sufficient and can be turned on or off as much as you like without the need to reconfigure (Unless you change ISP's or something).
Try to get a gateway with a firewall and WEP encryption.
Something like a Linksys WAG54G would do nicely.
Without more info about what you intend, it's difficult to be more specific.
Hope this helps.
AMD Athlon XP2200+
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Benq FP767-12 17" 12ms
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