I have a HP Pavilion dv5-1004nr. It's a refrub, paid about $500 for it. I use Traktor Scratch Duo.
Specs:
AMD Turion X2 Ultra 2.1 ghz
4 gigs of DDR2
Windows 7 64bit
Windows Experience Index
Processor 4.9
Ram 5.9
Desktop Aero Performance 4.1
3D Gaming 5.2
Disk data transfer 5.1
I run Traktor with Aero on and sometimes even have Yahoo Messenger open in the background. The only hiccups I've ever had were because the virus scan was running or I forgot to change the power options from Battery Saver back to High Performance. I only use it for basic playback of MP3s though, no effects, no key correction, maybe the occasional looping.. it's just a pain utilizing those features without a midi controller. So you don't need the latest and greatest to get good, reliable playback. if it's a dedicated machine for Djing you could get away with something fairly inexpensive. I think this machine could handle the usage of the bells and whistles if I needed it to and I still have lots of things I could disable to get better performance out of it (disabling Aero, killing other background tasks, reloading it with XP, etc.) I would recommend installing a fresh copy of an OS, don't use the one from the factory, they load all kinds of bullshit on it you don't need.