I strongly advice to build computers for yourself first, next consider the complete picture when you sell computers to others, first think that you need to work out some garantee agreement, otherwise they will think you give them lifetime warranty. Also consider the sort of clients you are going to get, do they ***** up their box with viruses every week and are you going to offer software support too? Do you know enough about the hardware market to know chipsets and their reputation about stability, speed, ... don't believe all reviews cause a lot are biased. Are you going to search out good power supplies or just going to purchase the first cheap 400w psu you see? Are these boxes custom built for clients or are you going to built thme your self and sell them as is? Did you look at the market? If you look around then you are going to see that a lot of brands exist and that they probably are going to offer boxes that are very close to your price counting parts. A lot of questions, and yep you said it's only for fun, therefor these questions to illustrate that just for fun can be tricky.

You people speak about 2.4ghz Athlons ??? you mean real speeds or the advertised speeds? I hope you don't wan't to sell 'new' boxes with an AthlonXP cpu (socket A, s462)??? That's completly old, even the s754 is 'low budget' now. A new made high end box based on a AMD cpu should have Athlon64 s939, FX or normal, or the AthlonX2 dual core cpu.