you can do a mix of wireless and wired if you want. The max limit for cat5 without some form of repeater is 100 meters (that should be more than enough to cover your house, no matter how big it is ;) ) Wireless will be slower than a typical wired environment, but it will still be much faster than your DSL connection.
the only problem you may have with wire, is getting it from the basement to the other PC. There shouldn't be any loss. Athough I try to avoid running the cat5 along any power lines, since it's unsheilded.
I've gone too far and need to move on!