Presumably you're going to need to use Photoshop on the hoof, so you're looking at something with a fast hard drive and plenty of ram - I'd say don't go lower than 1GB of ram for starters. And a fast processor, plenty of storage space (presume you're using uncompressed tiffs or raw files).
You'll also want good battery life - the last thing you want is the lappy dying on you when you've got some agency or newsdesk howling at you. And a wireless card to connect to hotspots, and you might also consider some kind of GSM/GPRS data card, too, for when you're out of reach of a hotspot.
Yes, you can get combo DVD/CD writers.
What's your budget? It's going to cost ...