I think that most on-board features (audio, LAN, wifi) put a load on the CPU more than stand-alone cards.
Also, a stand-alone card will probably have a better signal strength than the onboard.
I am getting the P5B deluxe
Anyway, I'm not using wifi. wired LAN is faster and more reliable
One thing I noticed with the P5B-WIFI is that it comes with a wired antenna about 1m long, this would be great IMO as Use a dlink PCI card that has a screw on hard antenna about 70mm long and as my PC sits under a hardwood desk the reception isnt as good as I would like it to be, I plan to get the WiFi version of the P5B and then put the PCI card in another PC that's a bit closer to my router.
the p5b wifi also has more fan power sockets. good if your case has 5 holes for fans,
The wifi model gives you one less occupied slot.
Don't bother with descrete cards unless you really care about the performance. Even with my old P4 I didn't notice a performance impact.