I don't know about a netbook but I've run vms well-enough (without crashing) on a laptop w/dual core cpu and 3GB of memory.  The same laptop now has 4GB and VMs run fine.

Aside from that, if you were thinking of doing any wireless hacking it would be good to have an adapter capable of monitor mode (passive scanning) and packet injection.  This will depend on both the wireless adapter and the available driver for your system.  If you want this capability, it may be safest to get one of the usb-attached alfa adapters.  I'm currently running one under ubuntu and it works fine.

Another computer would be a plus, though you can always test against a vm.  I've been able to pick up machines suitable as targets for $100-150.

Really, any sort of equipment could be a good tool for learning.  If you picked up a used machine at a flea market or yard sale, then you could boot it up under a live cd, maybe something with forensics tools like Caine, and examine it for data or malware.  Then afterward, it could be wiped, given a fresh install and recycled as a pentest target.

Hope this helps.