XL may still feel small to you. I ride an XL and only clock in at 6'1". You might have to swap some components around to get the *pit right for your height. The important number for you when sizing frames is the ETT (effective top tube), that's an imaginary line drawn from the center of the head tube to the center of the seat post. Frames are sized by the distance from the bottom bracket to the top tube but realistically that measurement doesn't matter so long as you can comfortably straddle the frame (you won't have a problem on any frame that's not a custom built monstrosity at your height). The important thing is the ability to locate the bars in a comfortable position which is why getting the top tube is the most important thing. If you have to use a ridiculously long stem to make a too small frame fit you'll compromise handling.
It's better to buy used than buy cheap. A new bike with cheap components is not a good bike. If you can afford a new quality bike then that's even better because it will come with a warranty and support from your LBS (local bike shop).
I'm not sure what shops around here carry Diamondback (none that I can think of except the 'sports stores'). The brand makes some good bikes on the high end but they also make a lot of junk bikes that are sold through places like Sports Authority and Dicks. They're perfectly serviceable bikes for casual rides and rail trails but they will fail when repeatedly subjected to the stresses of off-road riding especially with a heavier rider. Don't buy from Sports Authority or Dicks. Your money will go a lot farther on the used market or if you really want new the Sette bikes from Pricepoint.com are good quality with good components or bikesdirect.com which has decent frames but typically much better components that you could get elsewhere for the money (they do go cheap on the stuff you don't look at like the bottom bracket and the bars,stem,seatpost,etc but it's usually just heavy not unreliable). But if you order one it would be good to find someone who knows how to tune it up. Anyone with decent mechanical aptitude can put it together but there are lots of little tips and tricks for setting it up that aren't straightforward or obvious.
edit : *pit = c0ckpit, stupid censoring. I like how 'Dicks' doesn't get censored though.