Lower price
Price difference may not be as great as you are calculating. After doing TONS of research before buying my Powerbook, I stumbled across a place in New York that is an Apple Authorized Reseller with the 1.25 GHz 'book at $200 below MSRP. They also allow custom configured orders on powerbooks over the phone or via email. If you're outside of NY, then you also avoid sales tax which would be another $200 or so. Check out jumpsystems.com.
There are lots of little things with the Powerbook that will start to add up -- details that Apple pays attention to that the big PC manufacturers ignore. The power adapter on the Powerbook is sensational. It's small and light with built-in cable management for easy transporting. The plug on the power brick can easily be swapped out for international plugs if you but the international travel kit. You can keep it small and light, or add an extention with grounding if you need more cord. The power adapter on the HP is cheap and klunky. The new Powerbooks have keys that light up in low lighting. Not sure if this is relevant for your uses, but I think it's great. Adjusting volume and brightness is a single key press on the Powerbook, whereas on the HP it will require a key combination. There's no ugly latch hook that protrudes from the lid on the Powerbook, whereas the HP has this big fat plastic hook . A friend who experienced my Powerbook next to his Toshiba said it's like he has a Hyundai and I have a Porche. The design, quality materials, and attention to small details makes the Powerbook a standout.
Unless you really NEED to do Windows, I would recommend the Powerbook. Your everyday interaction with it will be bliss.