It breaks down like this, I can spend $1600 for a great mid level rig or $4,000 on high end one. The $1600 computer is designed to last me 18 months and then I get a new one. The $4,000 rig will have to last 3-4 years at least to be worth it money wise. The second $1600 computer will be as fast or faster than the high end workstation from 18 months before.
I can tell you that a hand built (by you @ newegg) will not be THAT much slower than a $4,000 computer to begin with. More importantly, what is slower is ok to be slower. If you're doing particle/fluid simulations, or 3d rendering, then the slowness will cost you in time, which is just inconvienient not costing you money or frags. Small freelance gigs will be manageable with the newish proc and won't be significantly slower.
The difference will be ram size and proc count, not ram or proc speed as much, esp if you get into even minor overclocking. The sad truth is hardware depreciates to "nothing" so fast whatever you spend on a computer is money being slowly thrown away. Keep it at midlevel and keep it fresh, and you'll be fast and always up to date with the most bang for your buck.
I'll end on this story. My friend got a $4,000 workstation at the same time I got my $1600 one. His was 8 core (2 quad @ $1500 each) His computer is fast and so is mine. One night his one proc burned out due to a cpu fan lock up. He didn't have a warentee and didn't buy a whole system from a company like Dell, totally his responsibility. He couldn't afford another $1500 proc and the other didn't work with just one. He had to buy 2 new procs at a lower speed in order to get back up and running. Another reason to buy a computer where any fail component is easily replaced.
EDIT/UPDATE:
newegg.com sponsors a build of the month ($1500) for CPU magazine. A great reference for what to get in the ever changing DIY Computer build.
