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.

What sort of hacking are you interested in?  Network, web, system?

Web hacking really only requires a tool like Burp Proxy which is not too resource intensive.

Network hacking requires that you run a lot of VMs, unless you have the actual systems to hack, which would require a lot of memory.  Network and system hacking can also require password cracking, which means you'll need a relatively fast processor.

If you're not sure what kind of hacking you're interested in, you might want to try eCPPT to get exposed to different areas of security.

If it's web application security, and exploit development (for e.g., programs), then you don't need anything special. I've been using what most people consider outdated and slow tech for years, but if you run Linux and Windows (XP) in Dual Boot you'll be fine with even an old computer / laptop or a current netbook.

If you're going do wireless attacks then you just one with a decent chipset, or a laptop that supports pcmcia cards and then such a card with a decent chipset (like some atheros models).

You can also install BackTrack to a USB key using UNetBootin, and it works quite well on a kingston data traveller 2.0 (USB 2.0).

It's wrong to make the assumption that you need high tech equipment, but having a good CPU with hardware virtualization as mentioned earlier, makes life easier, including having a couple of GB ram 2-3 minimum, but it is possible to cope with 1GB and a 1.6GHz single-core Intel Celeron M processor, I used that for almost 3 years and still use it on occasion.

After all, a text editor or a browser doesn't become that much faster, if you're reviewing code, or manually testing a normal website  

Just sharing my 2 cents again as I'd like to let beginners that may read this know, that you don't really need high tech equipment for hacking, you need a high tech brain  (If you want to be good that is, almost anyone can be taught to hack and do the basics.)

the only reason i can think of to go all out on a new rig is brute forcing. A decent processor and GPU (with cuda you can do some amazing things!) makes a huge difference in processing time while using for example john the ripper...still if the password is weak it will come up almost as quick on a netbook...