I would suggest that you use a 20" imac.

The best thing to do is:
upgrade the ram to the max
get the largest hard drive possible (I think right now its a 500 GB drive)
Get a 256 meg video card IF you intend to do some gaming or plan to perhaps add second monitor to it.

I don't think you really have to worry too much about the all in one design of the imac. Apple's quality and fit and finish is excellent and if you're really worried then just invest in apple care.