It's an interesting offer, noia! But I already know quite a few people who are willing to build whatever computer that I like for me. Besides, I can always teach it myself with the help of some friends. But thing is, I will have to rely on whatever machine that I will use, quite heavily. Thus, if it crashes after a month or so due to assembly errors or whatever, I want it to be fixed ASAP. (Preferably onsite too.) None of my friends can guarantee this kind of service. If I order it at some shop with 3 years guarantee, I should be safe for these kinds of situations.
To be honest, after I have a new PC, I will probably start disassembling and re-assembling the current system that I have. It would be a nice backup alternative. Maybe install Linux on it and use it as some fileserver. Once I have a new system, I can do whatever I like with the old one.
Dell offers a very good on-site support if need be. Many other computershops do the same, if you but a system that they have assembled. People who build their own systems often just have to work things out by themselves...
As I said, I am more into software than hardware. The hardware must be very reliable. And I do believe you can build a very fine system for me, but I don't think you'll fix it (for free) on site if something fails after a month...