If you need modem and WiFi, I would suggest keeping the internal modem und using a PCMCIA WiFi card.
Such cards a dead cheap, reception is as good as that of an diy internal solution.
Finding a good combo card is just not worth it.
That said, if you really want an internal WiFi solution:
WiFi b is slow (yet still faster than an analog modem), WEP encryption is very easy to break nowadays, so more and more networks switch to the advanced WPA/WPA2 encryption.
So if you use a card that can only handle WEP, you might not be able to connect to those networks.
The 240(X) hast very little room for the MiniPCI card. So you need a card with a low profile.
I would suggest neither of the antennae you linked to. There ist definitely no room for the first one, and the second one is just a cable without the actual antenna.
You should look for something like this:
http://cgi.ebay.com/HP-LAPTOP-NOTEBOOK- ... 20abead712 (I do not endorse this auction, it is merly an example what the antennae should look like)
You need two of those, they are usually sold as a pair (a black and a white cable). You should be able to get them for about 10 bucks (that's at lease the usual price here in Germany).
I installed them under the palmrest and keyboard of my 240x (now sold), reception was good enough.
Installing one or both antennae in the display may improve reception, but is a lot harder. The 240(x) is generally very tightly build.