Built me own for years - always had a view towards media applications (posh way of saying sat on my ar$e watching telly).
Current box is: Antec Fusion black, Innards:
Asus P5kl-vm 1394 mobo
2 gb ddr ram (could be four I suppose, but really don't need it)
Intel dual core E300 cpu - bit long in the tooth by todays standards, but fine
2 x wd Caviar 500gb hdds - more or less 1 TB storage
1 x Hauppauge Nova-t 500 dual dvb-t tuner for Freeview
1 x Hauppauge HVR 1700 analogue tuner for Sky
1 x Nvidia 9600gt passive cooled gpu
1 x Creative xtreme music 5.1 soundcard feeding cambridge surround system
1 x LG Blu ray drive - forget which model
1 x powerline network adapter feeding 1x Linksys DMA2100 media centre extender in the kitchen - the best bit of all - 1 tiny box into the 2nd telly for the full media experience. Love it.
For gaming, definitely a decent gpu, definitely a fast processor & definitely a decent cooling strategy - my Antec box was specifically deigned to compartmentalise the bits that get warm, so the psu lives in it's own chamber with it's own air supply, as does the optical drive & the main board area is served separately by two six inch fans that spin slowly & therefore quietly - important for a media box.
What it amounts to imho is that if you are after a machine for a specific purpose, choose two things wisely - the case & the motherboard. The case determines how much room for cooling the system & the motherboard will shape the ability to expand the system as your needs change - the bits on my box are not at all what was on it when I built it - as time has moved on, I have changed the cards & drives to suit my needs - lucky for me that mu mobo has let me do that.