I build all of mine. It used to be to save money, but now it's more on principle. Kinda like Ted Nugent going to a steak house...he'd rather kill it and grill it himself.
I'm sure your older son wants the big honkin' video card. I've seen the new 256 MB monsters for around $500(!). I'm only now getting to the point where my 32 MB card I bought 3 years ago is no longer doing the job.
Whatever you do, don't buy a Dell unless you like speaking to people from India.
Ain't progress great? Used to be "The kick-ass, off the shelf system will always cost $3000" ;)