Wireless is a good way to go but before you do that, consider buying a 5 port switch and placing it between the D-Link 500G and your computers. As the 500G is also a router, you will be able to connect both the PC and the Mac to the Internet at the same time. As both the machines will be plugged into the switch, you will have already established a network between the two.
The Airport express will suit you quite well and will communicate with Mac very easily. The device works on the 802.11b/g standard so almost any wireless card for the PC will work. Avoid getting a card that claims fantastic speeds, they do not mix well with anything that is not made by the same manufacturer. The WG-111 looks to be OK but if I had a spare PCI slot I would be tempted to use a PCI card instead. Not sure why, just me.
Hope some of this helps
P
I have a PC which is installed windows xp, and a notebook a macbook pro.. For internet i've already a D-link 500g.. so when i gonna use the internet on my laptop i need to disconnect my pc, and plug the cable at the laptop..
I'm wondering to buy a airport express base station, because its portable and i think it will work for me.. i wanna also network my pc and mac.. so i will need a wireless adapter to my pc right? ..the notebook has the airport already...
Is the airport express a good choice to network them? and about the wireless adapter to my pc.. a usb adapter like this http://reviews.cnet.com/Netgear_WG111_USB_Adapter/ o4505-3380_7-30791890.html should worth it right? or an pci wireless adapter is better?
Give me suggestions please..

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic