Limiting Factors for low light shoots

I'm just curious because I've had some problems with my D50 and 70-300mm VR lens when shooting in low light. My only experience with DSLRs is my D50 and I'm wondering what the biggest limiting factor is when shooting i low light.

I would think that a camera that can shoot very well at 800-1600 ISO would be best because you would be able to use the higher shutter speeds and higher F stops.

Does the lens itself play a role?

Any insight, I'd appreciate, Thanks!

the two factors would be the largest aperture on the lens and the highest ISO you'd be willing to use. usually i will compromise between the two to give myself enough DoF to work with depending on what im shooting and a high enough ISO to get the shutter speeds i need but not be too noisy.

And so its the job of the camera to be able to shoot in higher ISOs and the lens to provide a wide aperture?

yep, its a combination of the two. if you had a lens that could only do up to f/4 but had a camera with nice high iso performance, that wouldnt be much of a problem. but if your camera doesnt do high iso very well, youll want a lens with wider aperture. ideally, youd have both.

and a tripod

Lens aperture, f/2.8 or better. Go to f/1.2 and you're paying a high price !
ISO speed, the higher the better, but TANSTAAFL so you lose quality.
Camera AF speed (1D good, 20D bad)
Viewfinder brightness and size if you manual focus (1D good, 20D bad)

* There Ain't No Such Thing As A Free Lunch