I haven't had a desktop for a few years, but set one up yesterday. Its more for work than gaming, and is intended as an 'always on' machine that will supplement the chromebook.
Hardware:
HP Microserver Chassis (Only £130 with cashback - bargain!)
Extra 4Gb RAM
USB Sound card.
The chassis can support up to 8Gb RAM and 4 SATA disks, has low power consumption and is nearly silent. The only thing that isn't ideal is the non-upgradable, 1.6GHz CPU but I can live with that thanks to the compelling price. The £130 included a 250Gb disk and 1Gb RAM to get started.
I was going to add a flash drive for readyboost but it isn't worth it as this thing won't be rebooted routinely. I will be adding a widescreen monitor that is roughly the same proportions as the chromebook screen to make VNC a nicer experience - my current LCD monitor is 4:3.
Software of note:
Win 7 Pro
ThinVNC (remote desktop from the chromebook)
Subsonic (stream music to chromebook/android)
Dropbox (to sync local code files so I can edit them on the chromebook using sourcekit)
Drupal CMS (For work)