If the CPU and mobo are all they offer for $600 I'd advice to look somewhere else.
Newegg.com should be a pretty cool spot to pick up hardware. I think they have 'combo' offers for motherboard/cpu.
I have to say I'm not really familiar with the US webshops but usually you can purchase hand-crafted boxes from them for an extremely reasonable price.
The idea is to shop the parts you want and let them put it all together and mail it to you. Anyway, a combo that consists 3400+ A64, 512 ram, 120Gb harddrive, 17" crt screen, keyboard, mouse, dvd-burner, box, powersupply etc. can be shopped for $1100 ready built.
About $800 without the screen..
So it pays to look through a few sites. I'm sure some of the US residents can give better advice on where to go.
Err.. you ARE from the states right?