I'm not sure I fully understand what you are proposing.
You have a wireless router/switch and DSL modem in the basement with a bunch of PCs wired to the router.
You have some PCs upstairs hubbed together.
I may be missing something - you don't mention any wireless PCs.
By AP - you mean a wireless Access Point ?
I think you hope an AP upstairs might communicate wireless to the router in the basement. Fraid not unless the router is not being used by any wireless PCs AND has the ability to be a wireless bridge (unlikely but there are some that can).
There are two ways I can think of to join the two groups of PCs and share the DSL connection.
1. The obvious - a wired link from the router's switch to a hub (switch is better) upstairs.
2. A wireless bridge. There are some new wireless devices that can be set in bridge mode to do the very thing you want to do.
An example is the Linksys WET11. I am not sure if this might communicate with your router - it is quite new.
This diagram indicates it might do what you want:
http://www.linksys.com/products/disp...id=432&grid=22
Worth calling Linksys and asking them.
See the WET11 here:
http://www.linksys.com/products/prod...id=22&prid=432
Downside...cost ~$100 each last time I looked.
Bear in mind that a wireless bridge may be a bottleneck when both networks are talking to each other over a half duplex 11Mb link (more like 4Mb in real throput).
I would go for a 100Mb wired link unless that is impossible.