The thing is the machines on general sale are good all-rounders that will do for a bit of video/photo editing, the graphics in games will be 'OK'. Memory in all respects is cheap with 1TB drives available for less than £70 and 1Gb of DDR2 is around the £15 mark.
Most people who've built for them selves have an opinion about motherboards, as such its usually a personal choice. For example given the choice I'd go for Asus, then DFI, then MSI... But when you buy a pre-built system, you don't normally have a choice.
Gaming depends on the games. WoW, Guild Wars, EVA and other MMORPGs don't need much in the way of modern graphics processing power, but do benefit from a decent graphics card. CoD4, GRID etc demand at least an nVidia 8800 or ATI 3**0
Video editing and encoding benefits from multiple cores much more than most other programs. If this is the main use I'd recommend Intel's Q6600 or one of AMD's X4 Phenoms.
Hard disks; stick to Samsung, Western Digital or Seagate.
Memory; Apart from recommending heat sunk RAM, just buy the right one for your system. Note that the different types are not interchangeable, i.e. DDR will not fit in a DDR2 motherboard.
Despite my Dell comment the Medion deal at Tesco is a hard deal to beat for a pre-built Q6600, 1Tb, blu-ray, 3Gb RAM system:
Medion Q6600 3GB PC Base Unit with Blu-Ray - Tesco.Direct