Tarquinh:Why not just be done with it and buy a mac?
Games
In general I still view this as a marketplace which is yet to develop a properly affordable do-it-all solution.
Eliminate the games and there is a multitude of Vista, NAS and Windows Home Server boxes that will meet your needs (yes even-Macs
).
If you must play the latest games then as mentioned elsewhere noise, heat, power and size become a major issue.
Personally I'd use a separate PC for each task. The media PC can then use low power consumption components so it drinks less from the mains when playing music and stops a 200W+ gfx card costing you when it's not in use.
If you want to have your cake and eat it (without full-blown water cooling) then here's some tips:
1. A case running 120mm fans.
2. Fans with a noise rating sub 27db.
3. Gfx card with vapour-chamber cooling.
4. Look for explicitly quiet hard drives. Don't buy 10k Raptors for speed only to be deafened by them spooling.
5. Use eSata for external backups (fast) unless you can afford a Blueray burner.
6. CPU's built with the smallest process are more heat efficient so you can use slower fan speeds or posibly even a passive cooler.
There's loads more to consider but as others have suggested you may need to narrow down your requirement a little further first.