The basic principle is to get as much memory, as big a hard drive and as fast a processor as your budget will allow !
My personal recommendation is to look at AMD Athlon 64 Skt 939 processors, 3700+ as a minimum and if you can afford a dual core, go for it. As for a laptop, AMD Turion processors are good, but Pentium 4 M's still take a lot of beating.
Hard drive prices are tumbling so get a good, fast SATA drive, 250GB minimum.
Try for 2Gb of memory, 1GB as an absolute minimum.
Don't skimp on your graphics card. A nice Radeon or NVidia will be ok, preferably PCI-X16.
Onboard sound should be ok, but make sure it's capable of at least 5.1 surround if you're watching DVD's. It might pay to get a decent card instead.
That should get the ball rolling.
Oh, and whatever you do, DON'T buy it from Pissy Werld, or DICK-sons

My advice is to get it custom built from a trusted local specialist. That way, you only pay for what you need and the service will be better
Rob
