pete i've got a sigma 17-70 and i love it. it's quite fast for a general zoom lens (2.8-4.5) which means you can get some nice wide shots in darker conditions (like forests). It also has a macro feature which means it's good for up close. you can get RIGHT in people's faces – the dog, kids etc. and the build quality is nice and solid. i fully recommend it. I got mine for 140 on ebay second hand.
your 28-200 that you're looking at is not very wide so not that good for landscapes but if you prefer the longer reach for wildlife/sports etc its ok but remember it's not very fast either so you'll be bumping up the ISO to compensate for slow shutter speeds on cloudy days/indoors etc.
Here is a pic i got of my nephew with the 17-70