Bike Commuter in training
Join Date: Aug 2009
Location: Alexandria, VA
Bikes: '77 Centurion Semi-Pro, '88 Fuji Saratoga, '12 Jamis Sputnik, '13 Motobecane Fantom29 HT, '16 Motobecane Turino Pro Disc
If you haven't sold the Diamondback yet, I'd recommend that you continue to ride it in the same way that you intend to ride a new MTB. Make particular notes of where it performs okay, and where the bike becomes the limiting factor. Do this for one full riding season.
Then write back to the MTB forum, describing your riding style, preferred trails, height, age and weight, what you've ridden and what changes you believe you need to make. Lastly, you need to be realistic about your budget, because that number is affected by a lot of bike variables. They'll have specific recommendations that will help you.
Not knowing anything about you, but assuming that you're young and don't ride seriously challengine single track, you'll probably do fine on a 26" hardtail. Is that what you have now?