Another vote for the 70-200mm f/4..... truly fantastic value (especially used) and a lens that Nikon should be copying. It feels ridiculously small and light when you compare it to the usual (f/2.8) 70-200 zooms but it's sooooo easy to use will suit a small body like your 1000D perfectly.
The 75-300 USM IS (the black one, not the L version) is an excellent lens and pretty tough for what is essentially an uprated kit lens. Optically it'll never match something like a 70-200 f/2.8 but it costs a fraction. The IS is excellent on it, and stopped down a tad at the long end, it's sharpness can't be faulted. Wide open it's fine but not amazing. We run several of these at work and considering it's non-photographers using them, we get good shots, even at 300mm.
If you can stretch a bit higher then maybe have a look at the Sigma 100-300mm f/4. Bit of an underrated lens and built like a tank. I've seen these around the £500-£550 mark at used dealers like
MPB - maybe buying from the likes of Gumtree or fleabay could yeild a further saving. Think these are a grand new so a fresh copy might be out of the question. Optically they're great.
If it is focal length you want then you have to start looking at big bucks to get A) better overall image quality, and B) a faster maximum aperture of around f/4. I can't think of any zooms to match what focal length you already that have dramatically faster apertures - fast, long lenses tend to be primes and the fastest zoom is the one I'e mentioned already, the Sigma 100-300mm, or the Sigma 120-300mm f/2.8 (which is at least £1000 used).
The Sigma 120-400 f/4.5-5.6 seems to get a lot of attention and that probably falls in or around your budget. Think Tokina does a 80-400 with the same apertures but not sure what it's like optically.
If you want to get some good opinions on lenses have a look at
www.photozone.de. - very informative, reasoned reviews.
