Depends also what he's trying to run. There's a few titles out there that will bring a decent-spec machine to it's knees. FSX, Eldar Scrolls Oblivion and Armed Assault are good examples of contemporary titles that will not run maxed-out on many machines. Half-life 2 and Bioshock are the other end of the scale - they work well on a broad range of systems.
I'd go for at least a 4.4 ghz dual-core (although not all titles will use both cores), 2gb of RAM (I'm not a believer in 4..got some kit at work running games for the design workshops and I can't see a difference), and a mid-range GPU.
You don't need to spend a fortune. It's better to have it all balanced.