With the introduction of Core Duo and Core 2 Duo processors this situation has significantly changed and low end Intel processors began to offer impressive value. The Celeron 400 (code name Conroe-L) processors had only one 35W TDP core active and still managed to outperform previous generation 84W TDP Celerons.
Three generations have passed since then (Core 2 Duo, 1st and 2nd generation iCore) and here we are, Intel has just recently launched its latest desktop Celeron processor model, the G1610, which is a crippled version of the Ivy Bridge Core i3 processor. In this case "crippled" is a bit harsh term, because this ~$43 dual core chip offers an impressive performance: 2652 points in Passmark vs. the 4283 points obtained by the superior Core i3-3220 processor. Considering the $115 approximate price tag of the latter, we can safely say that the Celeron G1610 processor has a much better performance/price ratio. In fact the new low end dual core chip from Intel is more than enough for multimedia playback, office applications and even gaming, with one medium-speed PCI-Express 16X slot video card (like AMD Radeon 7770 or nVidia Geforce GTX 650).
With the mentioned components I'm inclined not to recommend any mainboard or memory brand, because we're not talking about a high end machine, so the only important thing is to use two memory modules in the computer, in order to enable dual channel mode (which makes quite a difference).
The Intel Celeron G1610 processor needs just 55Watts to give its best, including the integrated graphics processor, which is an "Intel HD Graphics" unit, stripped of QuickSync and other Intel technologies. If one of the mentioned video cards is installed, then the total power consumption of the computer will be about 160W maximum, so any cheap computer case and 400W ATX power supply will suffice (assuming that you don't install any major components other than a classic hard drive, SSD and an optical drive).