I wonder what might be a good setup for a gaming PC that handles mostly retro games (so no recent hardware necessary). When I saw the introduction of the Steam Box controller, I thought this is the one controller that should handle also adventures, RPG's and most of other new Indie titles (Papers, please! etc.).

Is there any possibility to build such a system right now? Does anyone have a nice setup running that can be recommended?

My requirements would be:
- low power consumption
- low noise
- good performance and usability on a big TV (that also means usable from a sofa)

Any cheap HTPC or low-end laptop, which has HDMI? And use a wireless keyboard + mouse/trackball with it?

I occasionally have my (high-end) laptop connected to the TV with HDMI. I could see myself using a desktop PC with wireless controllers the same way, as long as the HDMI cable is long enough from the desktop PC to the TV. I don't recall what is the practical maximum with one cable without any repeaters or such.

I think I have a 3 meter or a 5 meter HDMI cable (sorry can't recall which one), after I read somewhere that the HDMI cable shouldn't be longer than that. At least this cable displays good and sharp picture on my TV (1920x1080).

Are there nowadays cheap wireless ways to transfer PC video to HDTV, without lag? I have a home theater stereo system which can receive audio (also from PCs, tablets etc.) with Bluetooth, but it isn't very good for gaming because there is lag in the audio due to Bluetooth. But it is great for listening to music (e.g. playing from an Android tablet, maybe even smartphones), as there lag doesn't matter.

I've been looking at Intel NUC to hook up to my TV.
Small PCs with HDMI output, somewhat low power consumption (could be lower though) and high performance.
The major downside on these, is the price, they don't come cheap.

Better shell out enough money to get a dedicated rather than integrated GPU for better compatibility with older games. Yes, yes, I know that the OP only mentioned retro games, but you might as well.

Playing PC games on a TV: you are doing it wrong, imo.

I don't know about any wireless solutions, but for what it's worth I use a 7.5 meter HDMI cable to connect my PC to my TV and the picture quality is perfect. It really depends on the build quality and materials.

With extenders which actively boost the signal or use optical fibres you can reach lengths of up to 300 meters.

http://en.wikipedia.org/wiki/HDMI#Cables

As timppu sez, laptop should do the trick. Only concern is making sure your connections match, and HDMI makes that nearly a no-brainer these days. Er, and also making sure your GPU will put out the native rez for your TV (720 or 1080) else it may look funny / blurry.

This^

The former. Of course they might work - and probably will most of the time - but if there is a problem, it's usually one where there is no unofficial patch or other workaround way because the game simply will not work on Intel HD Graphics or the AMD equivalent, full stop.