Dell Dimension C521
The Dell Dimension C521 is a a slim machine from Dell, commonly shipped with Windows XP MCE. It is a great choice for a smaller myth front-end, while still having the flexibility of standard components (although half-height pci and pci-e slots).
The machine its self is relatively quiet, although you can hear fan and hard drive noise. The on-board video is nvidia, with analog d-sub out. They were shipped with a number of video card options. Mine had a great nvidia card with DVI, but the fan on the card made more noise than the rest of the system combined.
Half-Height PCIe opens up the doors to a number of myth-compatable, and inexpensive, video cards. My choice was an ATI Radeon 3450 with HDMI for $19.99 after rebate from newegg - cheap because it ships with a half-height bracket that won't fit in any system with out a hacksaw modification.
Sound refused to work at a normal volume in all current linux/myth combinations. MythDora, Fedora 11 alpha, Mythbuntu, and others. Sound is there, but you can't ever get it loud enough.
All-in-all a good solid myth box, front or back end, with the addition of a decent video card if you want digital video, and a sound card of some sorts (usb for basic stereo for $10, or a half-height pci card for anything good).
The beauty of an HDMI add-on video card is that it should also fix the sound problem, since it will have its own sound capability. Unfortunately, no one (that I've found) can actually get sound to work (in linux, works great in windows) on the Radeon 3450.