What kind of audio/video equipment will your friend be using? Not that the suggestions expressed already aren't good, there is a distinction to be made considering where the audio/video content is coming from. I do alot of video work (HD, miniDV, MPEG2, etc.), as well as audio encoding, and I'm on fairly generalized hardware.
Typically, all you need is firewire, alot of disk space (I have over 1.5 TB of storage and a small 100GB RAID0 with 3 Raptors for muxing), dual core (simultaneous audio and video encoding/decoding or for multithreaded encoding applications), and 2-4GB of memory (more always is better). A decent graphics card is all you need, but hardware MPEG playback is desirable. Good display is definately a plus. If doing capturing from TV or audio equipment, then you just need to look at capture cards to grab what you want - analog, digital, or HD. Most of these take care of audio also, but there are good standalone cards as well (the M-Audio line, as mentioned, is very good).
Good software to make use of all of it is where the real money is spent, but I'm assuming your friend can take care of that part (unless this is a new hobby for them or something).
I don't really support overclocking an audio/video system if it's for professional or family video archiving purposes - it can lead to frame dropping, corruption, sync problems, and other issues. If it's just for fun - go for it.