The Sigma 120-400 isn't a particularly good lens really with the addition of OS not compensating for the sub-par optics. Te 70-400 SSM is much much better. Actually the 70-400 SSM is probably the best x0-400 zoom out there, better than Canon and Nikon's offerings, let along Sigma.
Try to find a Sigma 50-500 or a Tamron 200-500 instead. Probably cheaper than a 120-400 and much better.
The 70-200 issue isn't as clear cut - from what I've seen off the Sony 70-200 SSM its very nice. But the difference between the Sigma and Tamron offerings isn't that big, although the Sony will be better wide open at 200mm than the Sigma, but not the Tamron.
Personally I'd seriously consider a Tamron 70-200 f/2.8 (it doesn't have the slow focusing issues of the Canon / Nikon variants as its body driven) or if you need to go 400mm get the Sony 70-400 SSM or a Sigma 50-500 / Tamron 200-500.
Or for 70-200 see if you can find a Minolta 80-200 f/2.8 HS APO which is a lovely lens - very compact and on a body with a fast motor like the A700, its focusing will outperform Sigma's HSM and Sony's SSM for speed.
