I've had purchased systems that were awesome, and others that were total trash. I've had homebuilt systems that were awesome, and also a couple that were trash due to hardware problems. Literally, the difference is in the quality of what is installed. I build my own as I want to know that the best hardware that I could afford was used. That said, my current system is:

1.5gig Intel P IV processor
Soyo Motherboard
256 meg DDR Ram
1.4 mb floppy (from old system)
10 gig IBM hard drive (master) (from old system)
100 gig Western Digital hard drive (slave)
54X CD rom drive (from old system)
24x10x40 CDRW drive
SIS AGP SVGA card
on board sound
Adaptec SCSI card
100 meg internal SCSI zip drive (from old system, try finding one of these now)
US Robotics 56K modem
10 base-T network card (from old system, need to upgrade home network)
All this is housed in a server grade full tower case with three cooling fans and vented sides

Other stuff includes:
17" monitor (untill the cost of 17" LCD displays comes down)
cordless keyboard
optical mouse
lexmark printer

I have a 350mhz PII in the living room serving jukebox duties.

The P IV machine replaced a 266 mhz Pentium, which will be resurrected to run my weather station that needs an old operating system to work properly.