The difference in RAM freq. really doesn't make any difference in any real world test i've seen, so don't worry about it.
Don't order fromt the US, unless you're in the US. The import duty will be nasty and add a whacking great percentage on the cost of the PC.
I'd err on the side of caution with the PSU if you intend to overclock. Go big, go Gold 80+ rated.
For overclocking, make sure that any PC you but has avaialability to overclock before ordering. Not all motherboards will let you do it. Please also note that overclocking will almost certainly invalidate a warranty. Assuming that the motherboard hasn;'t been locked out, there's no possible way to say if an overclock from 2.8ghz to 3.8ghz will be possible. Identical mobo's, RAM and processors have completely different overclock limits. If you can overclock then you may require an uprated CPU cooler.
One last thought: for the money you are spending and the performance you seem to want, think about getting an SSD to run games and apps from. It's worth infinitely more than a few extra MHz of RAM freq.