You can't go wrong with Antec or Enermax power supplies. The big name brands like Asus, Abit, Aopen, Giga-Byte, MSI are safe. Inexpensive boards from some manufacturers like Albatron are safe as well. Of course if you are going Intel and aren't planning on tweaking your PC much, I'd just go ahead and get a retail Intel board.
HL2 yearns for something better than my P4 2.4 @2.88 GHz + Radeon 9700 Pro. I get and average of ~40 fps at 1024x768 with medium settings on Counter-Strike: Source. You'll probably want a current graphics card like a Radeon X600 or GeForce FX 6800 GT at least...
The hardest part of building a PC is making sure all the little motherboard power, hdd, reset, etc. cables are inserted the right way. The second hardest part is making a perfectly <paper thin layer of thermal goop on your CPU.