Well, I mainly assemble PCs myself these days, but if I didn't want to, here is where I'd go to pay a little more for building, but get the exact components that I want at a reasonable price. In order:
* ARM Systems. Haven't ordered from here yet, but will soon. They are somewhat more expensive, but make their machines just whisper quiet. Their Stealth Quiet PC is fast and yet whisper quiet.
* "Proportable":http://www.proportable.com/. Bought a notebook from them. Reliable build to order. You really get exactly what components you want. They use ASUS as the chassis. The M5Ne is a very nice 3.3 pound note book. The M6 is a good desktop replacement. It's about $2K with a Pentium M 745 (1.8GHz) with 768MB memory, but the hard drive price for a 60 GB 2.5" drive is good.
* "JNCS":http://www.jncs.com/. Another vendor I've looked at but haven't bought from. Have good prices for ASUS build to order "M5N":http://www.jncs.com/static-temp/nb-as-m5n.php and also for motherboard, cpu combos.
* "Hard Drives Northwest":http://www.hdnw.com. Haven't bought there in a while but good as a local service and small business shop. Prices are OK, but they do assemble. No Athlon 64 systems on their site though 🙁