I want to run windows 8 as a guest os with virtualbox on some thinkpad (haven't bought one yet) running Ubuntu 12.04. Apart from virtualizing windows 8 (mostly just for use with the office suite app) my needs are very modest, I don't need much more than emacs and a browser. What I'd like to know is what kind of specs will be necessary to run windows 8 well as a vm, using the office apps. It would be a shame to waste money on overpowered hardware.

Are there any official guidelines from oracle or windows on this? Would this lenovo x220, for example, be sufficiently strong? The specs below were taken from this review.

Intel Core i5-2520M dual-core processor (2.5GHz, 3MB cache, 3.2GHz Turbo frequency)
Windows 7 Professional (64-bit)
12.5-inch Premium HD (1366 x 768) LED Backlit Display (IPS)
Intel Integrated HD Graphics
4GB DDR3 (1333MHz)
320GB Hitachi Travelstar hard drive (Z7K320)
Intel Centrino Advanced-N 6205 (Taylor Peak) 2x2 AGN wireless card
Intel 82579LM Gigabit Ethernet
720p High Definition webcam
Fingerprint reader
6-cell battery (63Wh) and optional slice battery (65Wh)
Dimensions: 12 (L) x 8.2 (W) x 0.5-1.5 (H) inches with 6-cell battery
Weight:
    3.5 pounds with 6-cell battery
    4.875 pounds with 6-cell battery and optional external battery slice
Price as configured: $1,299.00 (starting at $979.00)