For studying this should be enough. You don't need a high-end pc/ notebook for learning networking and similar.

For some aspects in the field of wireless security your wlan card and equipment is also very important. Recommended is a wlan card which supports beside some other things monitor mode.

In this thread you will find some recommendations about wlan cards.

I would also recommend a good wlan router such as the linksys wrt54gl.

For VMWare RAM is very important. On my previous laptop I had a 2GHz CPU with 512MB RAM and i could run one virtual machine without problems. However, I decided to upgrade RAM to 1GB and recognized a significant difference. If possible I would therefore first recommend to try upgrading your RAM before buying a new machine.
When I played around with WLAN stuff I also had the feeling that I could do better when working with two different machines (physically) then with virtual machines.