Hi,
It sounds like you've done a lot of reasearch on all the components that you've chosen. Staying with one distributor like you plan to do is a good idea, since you will soon become a valued customer, and it's just convenient like you saide!
Another thing you could do is allow customers to customize the systems that you have listed, using what you have as a guide. Perhaps charging a little more for helping the customer with their selection. I just think it will be a little off-putting to some people not having the option to customize their compter.
Here the main two points work through with your customers, before building. This is the kind of service computer companies charge a lot for.
1. Feasability Check; Find out what the computer will be used for (Gaming mostly by the sounds of it
), how much the customer can afford, and how long the customer is prepared to wait, simply because some very high end components could take a while to obtain.
2. Design; Picking out components to meet the customers requirements, recalculating the price to make sure your in the ball-park with the customer's budget.
I like your idea of charging for providing an overclocking service, maybe again a little more selection within that service might give the customer a little more to work with. For example, you could charge a small amount, and only overclock by say 500mhz for intel prescotts, maybe less for amd's. Run a few stability tests... Or the alternative is to find the absolute max, then take the chip back by ~50mhz. You could obviously charge a lot more for this, but obviously keep in mind you shouldn't charge too much, or you might exceed the price of a much faster processor.
Perhaps offering aftermarket cooling for the graphics card, Arctic cooling, or zalman. Either is a good choice.
Also, make sure you remember to include the OEM OS in the price, and maybe do an offer for free installation and setup of any other software. E.g. MS Office, AV programs etc.
When building custom computers to resell, you need to send the customer all the original packaging, along with the original invoices. since this will be needed if anything should need RMA'ing. Make sure you make a copy of the invoices for reference, in the case of the customer getting back in touch with a problem.
There's not really a lot more i can think of, you've done a great job getting some ideas down, now it's time to go and do it!
EDIT: Wow, that post got big fast!
~InfraRed