So a few months ago I decided I wanted to take a step up from my old point and shoot Nikon Coolpix camera and started looking around at digital SLRs. After figuring out my budget on what I wanted to spend on an entry-level camera I settled on the Nikon D40 with the standard 18-55mm lens that's included in the Nikon package. So far I've been extremely happy with it, although I'm still learning the ropes. I'm still tinkering with adjusting the shutter speed, aperature, white balance, etc to adjust the outcome of my photos.

In any case, I'm not super happy with the low-light performance of the lens I have, and want something that's going to give me some better performance, especially if I'm gonna try to snap some wildlife photos eventually. I'd also like to get a little more reach out of my lens, so I'm looking at something in the 200 to 300 mm range.

I'm looking at a Sigma 18-200 mm f/3.5-6.3D for $379 or a Sigma 55-200 mm f/4-5.6 for $149. I guess my question is, for the additional $200 am I going to get that much better performance in low light with the larger f-stop?

Also, what cameras/lenses is everyone else using that you've had good luck with?

Thanks!