Is there any can give me some advice about choosing camera?

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Now, I about to reach a tough decision. I am going to buy a single lens camera. I already have a nikon D80 and a Sigma 18-50 F2.8. And I am going to sell it out for a fresh technique loaded camera, but I dont have alot of budget. So I am going to take either Nikon D300, D200. Or I may have to chang whole my camerical system from Nikon to Cannon, I know it will incur hugely increasing of my budge. Is there anyone can give me some advice and help get rid of it? I really want to make it balance.