Is it a good idea to spend the extra £100-200 to get a laptop with Core2Duo/Core2/Hardcore2duo/whatever it's called?
Depends. If you're planning on doing little more than surf/email/poker then probably not. Gaming, it would.
Does a decent non-integrated graphics card make a big difference? Could you ever expect to play a decent 3D game on a non-hardcore laptop?
To gaming, yes. Having said that my laptop has an integrated Intel 915 128MB card and the kids are more than happy using it to play LAN Halo 2 and Lego Star Wars. Don't know if it would stand up to the very latest games though.
On the Dell website there are 4 laptops in the range, some have a 1 year warranty, others a 90 day, why do they do this? Did they make a bad batch? lol
They usually do the 90 day trick to make stuff look as good value as possible. You can usually downgrade the 1 year to make it cheaper. I've always done that and been prepared to rely on the Sale of Goods Act if required.