The store I use in the U.S. is samash.com.
Here is a link to an external recording interface that connects to a pc via usb. It's essentially an external sound card but it's manufactured specifically for recording, so it will yield higher quality audio tracks. This is a good example of a recording sound card (recording interface).
I'm not saying you can't record quality sound with something like a soundblaster, but the end result will be better with something that is designed to record. The lexicon interface has a microphone pre-amp, which means your friend can plug a mic right into it and start recording vocals, a guitar amp and drums. It also has a 1/4" input for recording a bass guitar direct, which means the bass plugs right into the interface (no amp). Another option is the stereo 1/4" inputs, that would let your friend use an external mixing board to add more microphones. Here's a diagram from another interface manufacturer that shows you how these things are generally hooked up. PC Recording Diagram
The more money he wants to spend, the more microphones and seperate channels can be operated at one time, extra controls, flexibility, expandability, and quality. Really the sky is the limit when it comes to audio recording. But he should also keep in mind audio quality doesn't just come from the audio interface (analog to digital converter), it also comes from the quality of the microphones, cables, instruments, and experience of the recordist. Best to set a budget based on the rest of the equipment being used, as to not bottleneck the recording rig. Recording interfaces go anywhere from $100-500+ so there are plenty of options.
A quality recording interface will also come with multi-tracking software, so there will not be a need to purchase software for recording unless the need arises for something with more capabilities.
In response to the other questions, I'll stay away from specifics as i'm not very up to date on current pc technology, but often pc recording involves running multiple programs at once, say a multi tracker, effects, and virtual instruments. So a quad core may be more beneficial in this area. Also plenty of memory and a fast hard drive. I'll stop here and let someone more qualified take over.
If you have any other questions or would like me to look at and compare some other recording interfaces that's fine.