should I even bother building this???

I have a Marineland Powerhead 660. It is rated 170gph. I was wondering if I should even bother building a second system with this powerhead. It would be cooling a Duron 700. I was thinking I'd use 3/8" hose this time..

what do you all think??

Sure!
Practice makes perfect, and you have time to get the new rig perfect (rushing=mistakes).
You never know when spare watercooling can come in handy, and you just might make a killer system that you'd like to carry onto the next PC in your future.

'Sides that, maybe one of your bud's will like it enough to fork up some $$ for it, letting you get some cool stuff for your own.

And there's nothing wrong with 3/8" systems. Mine's doing just fine!

Try it, you might be surprised how well it works.