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 I've been using what most people consider outdated and slow tech for years, but if you run Linux and Windows (XP) in Dual Boot you'll be fine with even an old computer / laptop or a current netbook.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.)