Daut, its not about about preferably 800MHz, its about what the motherboard requires and the higher end motherboards usually require no less than 800MHz. Another thing with the RAM, make sure you buy RAM cards that have either a latency of 4-4-4-12 or 3-4-3-9 (the lower the latency the faster the card)
The X6800 is the best CPU on the market hands down, there is no way you can go wrong.
As far as HDD, I was going to suggest a Raptor (which is very fast), but I dont know if 150GB would be enough for the kid. I say anything 300GB or higher will be a sufficient enough hard drive. Also, you want 4.16ms speed and 16MB cache.
If you want to go with something simple for the cooling components... I say get a ThermalRight SI-128 Heatsinker and attach a Delta Triple Blade High Speed fan to it. And for general case fans, you can get 2 more Delta Triple Blades or get Sanyo Denki's. Anything with 100+ CFM @ around 3000 RPM is a solid fan that will push around tons of air.
Just remember, if youre getting an ATI video card, you MUST get a motherboard that supports Crossfire and not SLI.
With the power supply, get a 600W power supply... this computer is going to be a beast, so you need a proper power supply.