It doesn't matter particularly. The WGR614 uses the standard 802.11g protocol (54Mbps) so any basic Wireless Network Adapter, which can be either USB or an internal PCI card, will do the job.
I have Belkin USB and PCI wireless adapters which connect reliably to a Netgear WGR614 router.
Belkin USB WiFi adapter click here=
Belkin PCI WiFi adapter click here