I personally don't like full-scale PCs for audio--there's just too much overhead from the software that isn't necessary and might mess up the audio without you realizing it (case in point: kmixer). Also, making a full-scale sound card is a whole lot harder than making a DAC. You have to deal with the PCI bus, all the digital noise inside a full-scale PC, and the drivers for the DAC. Quite a nightmare for any DIYer. I'm quite certain it's virtually impossible to do some crazy modification to the motherboard in order to accomplish this.
Either way, a 2Ghz CPU is about 10000x more processing power and complexity than you'd need to do this sort of stuff. It's probably easier to just design a DAP from scratch. The rockbox project actually has most of the schematics for the iriver on its site--it shouldn't be all that difficult to make a copy of the iriver using a bigger board and mostly thru-hole components, though it wouldn't be fun to solder/debug 100+ SMT processor pins. Most of the work would probably lie in the layout and modifying the rockbox firmware for the modified components (no li-ion power controller, different DAC, etc. . .). I've been planning to do this sort of DIY audiophile DAP this summer, although I was going to jump directly to an ultraportable design.
btw, the MCF5250 happens to come in a LQFP package, unlike some of the other coldfire chips I was looking at, which means it's accessible to a lot more than the 10 DIYers who have a toaster-oven-bga reflow system set up.
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