I know some people with Mk2 Astras have had problems with using polybushes on the front because they have a habit of wearing part of the chassis, or subframe away. It'll depend on how much the front suspension layout was changed for the Mk3s after the Mk2s. If they are basically the same, then standard GM items are the recommended formula for the front. The rear should be fine though.
If you do discover you can use them without having the chassis worn away, then go ahead. They do make the car feel tighter on the road. As for extra road noise, you hardly notice it at all
Personally, I'd stick with new/good standard bushes on the lower arms, even though, the Mk3 has a different setup to the Mk2.
That's what I do although I also have poly Anti-Roll bar bushes in a fetching shade of yellow.
If you do want poly bushes, Rally Design in Faversham is THE cheapest place to buy them.