The only thing ypu need to worry about are a compatable network card and graphics card, the rest is up to you really and what you can afford to pay for memory, CPU etc etc...
I am sure there is a list of compatable cards on the Offensive Security website somewhere.
The laptop you have just now seems good enough, infact it's not too dissimilar from the setup I have... why don't you just buy a new network card for that laptop or even cheaper a USB Dongle?



