My next purchase, I hope (regardless of the lack of WAF) will be a compact PC for playing Steam games.
What I want is a compact form factor, a la Mac mini, or otherwise as small as possible, PC that I can use to play, ahem, Train Simulator 2013 via Steam.
I've been mac only for more than 15 years so I really have no clue about Windows. I have an Xbox but there is not a similar game, unfortunately. So I may have to buy a new device, damn ;)

The requirements are
As small form factor as possible, will be installed in entertainment cabinet.
HDMI connectivity as will be played on the TV
Powerful enough graphics to run at relatively high settings

I understand that games like this are relatively cheap, but have massive potential amounts of in game purchases, this is ok with me.

Main system requirements for the game are:

OS:Windows? XP / Vista / 7 / 8
Processor:Processor: 2.8 GHz Core 2 Duo (3.2 GHz Core 2 Duo recommended), AMD Athlon MP (multiprocessor variant or comparable processors)
Memory:2 GB RAM
Graphics:512 MB with Pixel Shader 3.0 (AGP PCIe only)
DirectX?:9.0c
Hard Drive:6 GB HD space
Sound:Direct X 9.0c compatible

Any suggestions as where to start?

Thanks,
Joseph