HP MediaSmart EX487
For those that want a pre-built WHS box, HP has an offering that you may want to look at.
For those who care most about benchmarks, we found that the EX487 is a serious step-up in speed and reliability, having used it for a full month before reviewing it. A massive collection of 7,000 documents (about 40GB) took 1 hour and 5 min to copy, or about 30 minutes faster than a similar transfer to a Western Digital hard drive. Meanwhile, a photo collection of 4,242 files (1.8 GB worth in 25 folders) took 1 Min 5 seconds; the same test took 3 minutes 30 seconds to copy to a home-built Windows Home Server. A video transfer of 9 complete theatrical movies (8GB) took 2 min 30 seconds to copy, compared to 3 minutes 45 seconds to the home-built server. In fact, most tests we performed ran at least a minute faster compared to other servers and USB hard disk drives we tested.