For a longer battery life, get a smaller screen. A 15.4" screen is likely to have reasonably good battery life these days. Any larger and you'll probably haemorrhage battery time.
For the CPU, I'd get a dual-core. I'm loyal to AMD's processors, but I hate to admit that Intel's dual-cores are very competitive in price and performance. I would never buy an Intel single core, ever.
For a not-too-over-the-top graphics card, have a look at lower-end 8-series, or higher-end 7-series nVidia cards. I wouldn't buy anything from AMD/ATI in the GPU department just yet. If you plan to play games on it, don't get less than 256MB, and if you can squeeze it out of your budget, a card with a "GT" (or GTS/GTX

) on the end is the way to go.
A note on hard drives - you'll pay a lot more for an external drive than for an internal drive of the same capacity. I would go for the largest internal drive capacity you can afford.
Wireless and DVD burners are pretty standard these days.
Personally I wouldn't spend my money on XP, and there isn't a thing they can do about that. Read into that however you will.
