For pricing check out
www.newegg.com if you live in the US. Brand wise - for AMD an ASUS board on an nForce4 chipset. For Intel either an Intel or ASUS board on an Intel chipset (newest are the 9xx's). Antec has good case/power supply combos. Corsair for RAM. LITE-ON or NEC for opticals, and Western Digital or Seagate for hard drives. Those are my picks.
You can't beat the value of a $300 Dell with a *quality* custom build, but you might save more as you go towards the $800+ range. Unless they've got a deal going with a free monitor or something like that, then you'd be hard pressed to beat that.