My friend is building a new PC with the intent of recording audio, from a Yamaha Motif synthesizer. He already has an M-Audio Audiophile 2496 and a Asus P4 800 VM with an 800 MHz FSB. Will his choice of processor make any significant difference in recording quality? He is leaning towards getting a P4 2.8 GHz with 800 MHz FSB. Does anyone have any suggestions/recommendations on this processor?

AFAIK, processors play no part in recording quality. The processor is significant in the later work, as in mixing the audio, post-processing, compression, etc. Any modern processor should have no problems with this.

I'd be more worried about the harddrive speed, esp. if you are going to be recording multiple, hi-resolution tracks at once. In this case, RAID harddrives usually perform quite well.

Any modern CPU above 2GHz will do very well for home music production ... if your friend intends to use a lot of virtual instruments via ASIO at a time, some extra CPU speed will be of interest but personally, I cosider RAM as being more important. 1GB (some virtual instruments like e.g. Steinberg's 'The Grand' won't even work below 512MB) of low-latency, twin-banked DDR will be sufficient - along with a properly configured OS, of course.

Yes, no difference in recording quality. Processors which use more power however will be harder to run quietly.

The type of chipset the motherboard uses will be somewhat dependent on the CPU chosen. If you use AMD, you are very likely to have a VIA chipset (or nVidia). If you use a Pentium, then you have the option of using an Intel chipset.

I'm not saying one is better than the other, it's just something to consider.

I would definitely recommend either the Nvidia or Intel chipsets.
I have seen, and personally experienced, too many issues with Via chipsets.

I'm generally an AMD man -- I hold AMD stock, etc. For my latest PC I decided to go with a P4, though. (Asus P4C800E-Dlx)

One of the unexpected side benefits of the hyperthreading technology (essentially two virtual cpus instead of one) is that when encoding, only one of the two virtual cpus is tied up and I can use my computer for other things. I used to be very careful not to do anything else while ripping/encoding, but now I start up EAC and let it rip while I'm doing all the things I normally do. YMMV.

Neither encoding nor ripping should be adversely affected by running other tasks. You should simply see the process taking a little longer.