Just ordered the new DG834T 108mbps router from netgear and a couple of USB
adaptors. If I have 2 PCs both with 108mbps adaptors and 1 PDA with only
802.11b wifi connected to the router, will the 2 PCs still be able to
communicate at 108mbps or does the slow PDA "b" standard mean the whole
network runs at only 802.11b speeds?

Depends. Original 802.11g routers were slowed down by 11b client, but some
later ones are supposed to overcame that.

Knowing Netgear, you will have to upgrade the firmware 10 times before it
will work with semblance of reliability,
but if you are lucky, like me, it wont' work at all, and you will be able to
return it.
Regards,
Martin