Hi Jerry,
The path that you are currently traveling sounds very similar to the long and winding road which brought me to my current set-up

.
I started out with the 70-300mm VR (on a D700) and was quickly frustrated by the focusing issues it had it in the low light (forest) conditions in which it was being used. I also found mine to be less than tack sharp at max aperture, at anything over about 220mm :shrug:.
So, I wanted to upgrade, but didn't like the idea of spending nearly a grand for the Nikkor 300mm f/4, as it would only really be good for one or two things (wildlife/sports/long landscapes). Instead, I opted to get better image quality at '300mm' by going for the new Nikkor 70-200mm f/2.8 VRII and a crop-sensor camera body

(the D90). The perceived advantage here was that I would get a lens which I could use for all kinds of other stuff, as well as wildlife.
I quickly realised (as you have done) that 300mm focal length is nothing when it comes to 'birding' in the wild with nowhere to conceal yourself and so I tried to extend the reach of this kit with the addition of a Nikon 1.7x TC. Frankly, this wasn't a good combination (just as
Desantnik said it wouldn't be). Although my reach was now equivalent to 510mm, the once ultra-fast f/2.8 lens was slowed down and couldn't always focus accurately. What's worse, the images were about equal in sharpness to the (much cheaper) 70-300mm that I had started off with

.
Next step was to ditch the Nikon TC and go for the Kenko Pro300 DGX (1.4x) TC, largely for cost reasons, but also as it seems to work with more lenses than Nikon's version. Whilst I can compare it to a 1.4x Nikon TC, I would say that it's performance is very acceptable and the resultant images with the 70-200mm f/2.8 (now an f/4, effectively) where still pretty sharp.
But, I still wasn't happy with the lack of reach and the (frankly ridiculous amounts of) cropping, which I had to do for every bird shot I posted. More reach was the answer - but how to get it

!?
Well, going with
Desantnik's advice (again

), I just decided to go in at the deep end (well, for
my budget at least) and get myself the Nikkor 300mm f/4 AF-S prime. It arrived last week

!
This lens is frighteningly sharp, when used on it's own. I was amazed at how well it nailed the focus as well, even in lowish light. My impression so far is that it's not especially quick to focus and has an infuriating habit of hunting by going down to the closest distance (or to 3m if you flick the appropriate switch) and then back out again, if you are tracking a bird in an empty sky and you don't get the active AF sensor right on target (in AF-S mode). I'm tempted to say that it actually focuses quite slowly in most conditions, but I need to spend more time with it first.
With the Kenko TC mounted, the equivalent focal length goes to 630mm - finally, a respectable length for 'birding'

! Performance with the Kenko TC on appears to be very good indeed, from the sharpness point of view, although it slows down the AF to a level that is bordering on unsatisfactory, when used in anything other than clear daylight.
It's too early for me to know if this combination is really going to be 'good enough' for the kind of bird shots that I would like to get, as I'm still getting to grips with it. What I can be sure of though, is that I (personally) would never dream of putting a 2x Nikon TC on a 70-200mm zoom lens and expect to get either sharp shots or decent AF performance from that combination

. You'd kind of be taking a really great lens and effectively crippling it :nono:.
The combination of the 300mm f/4 AF-S and 1.4x TC (Kenko or Nikon would both fit within your budget

) would, I'm certain, give you far superior images and probably much better AF performance, too. Of course, for the ultimate in image quality and extendability, an old, battered Nikkor 300mm f/2.8 could be had for around 1,300-1,500GBP. These take TCs pretty well (or so I have read), but they are far, far bigger than the f/4 version and are not really hand-holdable for any length of time. Also, if you buy a really cheap one, you may get one with fungus, scratches, dust or other performance issues.
So, I can't really comment on the Sigma option, but I think that your first suggestion is a good one. Good luck with deciding

.