Athlon processors have very powerful legacy floating point execution units (floating point numbers are non-integer numbers, i.e., numbers that contain digits to the right of the decimal place). The Pentium 4 does not. It relies on software optimization for it's SSE2 instructions to be fast. Now, when software takes advantage of this, the P4 is a screaming chip (partially due to clock speed, but who cares... it's the end result that matters). When software relies on legacy "raw" floating point execution (also referred to as FP or FPU - as in floating point unit - or x87) then the P4 isn't so spiffy.
As far as I know, digital audio processing (applying effects, eq, etc.) requires lots of "raw" floating point math. Hence, the Athlon is well suited for this task.
You might check to see if the software you are interested in using is optimized for SSE2 instructions. If so, the P4 would be a very good choice as well... in that case, you probably couldn't go wrong either way.
Also, if you decide on a soundcard
other than one of the M-audio Delta cards, do just a bit of reading to make sure it doesn't have any compatibility problems with Via chipsets (a common chipset supplier for AMD Athlon processors).
Or, if you go with Dell, don't worry about it... they only sell Intel.
