The Sprint Power Vision Access Pack is what you need. Don't bother getting the higher priced ones because the extra features don't work with our phones.
I use a Socket E300 SDIO card that I've had for a couple of years on my 700wx. They have the P300 now, which looks like it only adds 802.11g to the mix:
http://www.socketcom.com/pocketpc/WL6217-664.asp
Honestly, none of these devices are really fast enough to keep up with a 802.11b connection, so you don't HAVE to buy a g card. I think the fastest cards they have are 133x now, and that transfers at about 20mb/s. Most cards are 66x, which is about 11mbs, which is the speed of 802.11b. So even if you got a g card, you won't be able to physycially move that much over the SDIO bus.