I'm considering the switch from vinyl ad CD's to a laptop DJ setup. What would be the minimum RAM / processor requirement for a DJ software program like Traktor or PCDJ to work smoothly?
I doubt the specs will be an issue with modern computers, as long as you don't buy the absolute cheapest thing you can find. Even then it would probably work fine. IIRC, they've bumped up the Serato minimum requirements to reflect the reality that finding a computer that meets those specs should not be an issue. However SL1 (the older model) was actually a USB 1.1 device and would run ok (but not great) on some pretty ancient hardware (PPC macs included).
As a bare minimum I'd get a fast Core 2 Duo (T9400, etc) machine if looking used/refurbished or considering if what you already have is adequate. Probably more important than the specs is having a spare - even the most expensive computers fail or can be stolen or subject to physical and liquid damage. Consider what happens when your computer stops working mid gig and it's your only option.
If you're DJ'ing little house parties it doesn't matter, but if you're doing a 1000 person wedding or corporate event you'd be crazy to screw up the gig because you couldn't bother to bring a spare $200 computer. Yes, for $200 you can get a spare thinkpad T400 or something similar off a corporate lease and it will do everything you need and more. Uprade the harddrive and throw in 8 gigs of cheap DDR3 ram and you've got a great little work computer in case your primary machine fails.
Also, IMO, if you're trying to be a professional DJ a dedicated computer is essential. Keep your movies, family photos, bloatware assoiciated with any widgets you own on something else.