I'm seconding Daktech as a solid supplier, these guys have an absolutely ludicrous warranty. My department has a lot of their systems in service and they've done very well.
They're not bullshitting when they say a 7 year warranty. I've called them up about some dead CDROM drives in some 5-year-old celerons and they've sent over/dropped off replacements (I'm local). If they don't have the original part they send over something similar or slightly better.
They don't usually need to do this however, as they use good parts. I've taken some apart to find decent Antec cases/PSUs, Intel motherboards, non-proprietary form factors and parts.
The only problem is that they don't always have the bleeding edge stuff to spec your machines, but Dell will take you to town on that sort of spec anyways, so they're really not much better.