From previous blogs, I'd been having problems with the wireless not connecting to a WPA network. Unencrypted worked fine. Don't know about WEP since I don't run it.
It seemed to me it was possibly a wireless card problem, since running WPA involves the OS pushing code to the card, and I'd noticed that the card was 'unknown'. So after trying a few things, one day while browsing ebay I saw a genuine Apple Airport card, for the grand sum of £10. Bought it, fitted, and now I'm on WPA.
So my previous suggestion to get the Broadcom 1490 based card I would now revise to get a genuine Apple Airport, if you can. I'm sure that there are 3rd party ie non Apple branded cards that work OK with WPA.
Update: Found this link to some possible options on ebay