Think you'll likely find nearly as many opinions out there as there are options. Expect that the bottom line is that you'll be able to make whatever you choose work for you.
Personally, I went the Mac Mini route. I like it and it works for me -- but it is by no means the final word or only option. Running Snow Leopard on a 2009-era Mini, iTunes for archiving, Bit Perfect for playback, out USB 2.0 to an Ayre QB-9. I have been toying with getting a new Mini, maybe even with a power upgrade from the likes of Mojo Audio (which you can find here or on the web). A newer Mini (with Mountain Lion, faster processors, and 16GB of RAM) would likely be an upgrade. But I havent bothered, and am in no rush.
If youre looking for a total glut of info, check out the Music Server forum over at Computer Audiophile. Probably more than any one person could want or need in terms of information. The first variable you may want to address is which ecosystem/OS do you want to play in ie., Apple, which is swell, but can limiting and controlling, or non-Apple, which may be less plug-and-play but probably more flexible in the long run. What seems to be the main non-Apple option that has been picking up steam, and about which youll find a lot over at Computer Audiophile, are the various iterations of the Computer Audiophile Pocket Server (CAPS). They look real interesting, but I know next to nothing about them.
From there, the decisions tend to become more obvious, once pick the ecosystem. You probably dont need a whole lot of grunt (RAM/processing power) to make even high-def files run fine, but more is certainly better particularly if youre going to end up with a software front end that buffers into RAM for playback, which many do these days. In order to run above 24/96 files, if youre outputting via USB, youll need at least USB 2.0. On any new-ish computer, this will be a non-issue. But on mine, for example, you need some more aggressive configuring to ensure youre using the high speed v. the lower speed USB output, as the OS has both and assigns randomly.
Finally, think the only hard and fast rule is back things up. The only certainty in computer anything (which certainly includes audio) is that your drive will fail. Dont let that be your only copy. Enjoy.