for a gaming PC:
1: start with an socket 939 AMD Athlon64 CPU (the larger the L2 cache, the better)
2. a socket 939 MoBo (with nForce4 chipset and a PCI-e slot for video card)
3: some good memory (get at least 1GB, always in matched pairs for dual channel, i.e. two 512MB sticks)
4. a good 256MB PCI-e video card
5. a quality 400w (or higher) power supply (this is where most people are cheap, yet it is arguably the most important component on your entire system)
6. a 7200RPM (or faster) SATA or SATAII hard drive
7. a nice steel case (you can get a cheap case if you want, but it will be louder and not as sturdy)
8. DVD/CD rom drive (be aware that CD/DVD burners often times have slower read speeds then read-only drives)
that is your basic gaming PC. I have not mentioned any specific brands or models because we don't know what your budget is. If you have the money, you can build a killer system, if you're on a tighter budget, you will have to compromise/sacrifice on some things, but you can still build a decent system that will run all the newest games