Let me see if I have this right..
You have: 1 cable modem, 1 wireless router, 1 wireless NIC (USB) and two PCs.
If so, Does your wireless router also have a "switch component" ie. 2 or more rj-45 ports to plug in other pc's?
If it does then are plugging the cable modem into the wireless router and the closest PC into the wireless router also. Then across the house the wireless signal is being picked up by the USB wireless NIC adapter. Have I got that all?
Speed. Well there's the good and the bad. I don't have Cable DSL, so perhaps your mileage will vary but here is what I've experienced:
Signal strength is king. There is probably some kind of signal monitor that will install with the configuration utility for the USB device. If your signal quality is poor then, if you've been disconnected by your service provider (witch may not happen via cable), the lag time introduced by the wireless side will not allow the mere launch of a browser to re-establish connection. This will mean going to the other PC and launching the browser there to establish a connection.
If your signal strength is good, however there should be no problem.
I have a PC using the internet across the house too, and with good signal strength there is no discernable difference. I use WEP encryption too.
I have had various results with programs. Most seem to run fine, but the only SQL server app I've tried choked over the wireless side. No idea why.
If you can get the signal strength nailed, your experience should be quite gratifying.