One of the biggest reasons I prefer to build my own rather than buy a "brand name built system" is you can get screwed over on the motherboard. You buy it then find out 6 months or 1 year later that your upgrade choices are severly limited or almost impossible.
My wife bought herself a Compaq something or other about 4 years ago. added some memory later, no problem. Added a new harddrive, no problem. I notice a few weeks ago a noise from her pc, look at it, the CPU fan was making the noise, I figured the bearings were starting to go out and need to replace the fan. Pull the heatsink and fan out and go shopping. Every where I went no one had that size fan, so I figured I would replace the heatsink and fan. the motherboard as a particular kind of mounting system for the heatsink, you cant find a generic heatsink for this pc you need to get an original Heatsink made for Compaq. Online searches I found several priced from $40 to 50 dollars. Stopped at a local PC repair shop in town and one of the guys had the exact same heatsink with a fan in a parts box. I gave him $5 for it.