So a 2nd wifi router in my BR wouldn't help? I was thinking it could pick up the signal from the router in the LR; then I would have a wired connection to my old desktop that way.
If you get one that can act as a bridge/repeater that would work, but I don't really see an advantage to doing that vs. just getting a PCIe or USB adapter for your PC, and the downside is that it'll probably be a little trickier to set up. There won't be any advantage in terms of latency, since the 2nd router is just another client as far as the router in the LR is concerned.
But it sounds like a decent PCIe adapter is easy enough to find - I see tons on newegg. I just wasn't sure an old motherboard would accept a wifi card.
As long as it has Windows 7 drivers you should be good to go. Needless to say, your odds of a satisfactory result go up if you avoid off-brand/generic gear.
As far as the motherboard itself is concerned, it's just another NIC. I've put PCI-based WiFi adapters in old Socket 754 motherboards, and they worked just fine.