So I have $2000 to spend on a lens. here's what i have so far: Nikon D300s Nikkor 50mm f/1.4 Nikkor 18-105 f/5.6-6.3 (not too fond of this, came with my D90) Nikon SB-900 So I do portraits and wildlife. the only choices that i have come up with so far is the Nikkor 70-200mm f/2.8 VR II (when it comes out) & a 1.7x tele. , or the Sigma 150-500 OS. any suggestions?