Just buy a wireless router if you have broadband. 98se and later have internet connection sharing but you'd need to get to the wireless computer anyway with the second NIC. just buy a soho wireless router and read the manual. It's fairly easy to setup, Many are ready to go out of the box.
I have a wireless router on the 2nd floor. Basically, I have a group of PCs on the 2nd floor and another group on the 1st floor. Would the best way to network them all together would be to get another router for floor 1 then wire the 1st floor PCs to that router? I don't want to have to put a wireless card into each PC
Well you wouldn't buy a router. Actually you could, you just wouldn't use the routing functionality. You need a bridge as mentioned in the other post reply however I would recommend strongly against using windows as a bridge. Depends on what kind of Router/AP you have in place as to what you could or would buy. If your router is WDS compatible you'd only need another wds bridge compatible device or if it is an older router/XP you'll need to stick with the same vendor. Bridging before WDS is proprietary. May work, may not.