Hi EZ... a few thoughts from me here.
1) Make sure you get a Core Duo processor... the battery consumption will be a bit higher than say Turion.. but the speed is at least 2x as fast - I wouldn't purchase anything else these days.
2) 2GB will be plenty of memory for PS - sure the more the better but 2GB is a great point to be at. As mentioned above, just be sure that the laptop has at least 4 memory slots... and only 2 (or even better 1) are filled with the 2 GB... some cheapo companies will actually stick 4 older 512 in there for 2GB!
3) 12.1" displays are small... but you will notice that the lack of screen space with CS3. If you don't mind the small extra weight - I'd recommend 14" minimum.. and 15" maximum. 17"/19" are only really useful as desktop replacements.. once you get that big, carrying them around (and even using them) gets unwieldy except on a big desk.
4) I don't usually say this... but... get the extra insurance for at least 2 years. Laptops are notorious for breaking.. and unlike a desktop where the warranties are useless because you can easily replace a part with another one... you'll be out of luck in many cases if you say.. drop your laptop and it breaks.. or it won't boot up, etc.
5) If you can........... get XP instead of Vista. Vista is still a memory hog and has some problems while XP is almost perfect at this point. Most new laptops in the big stores only ship with Vista.. but you might find an older closeout with XP. Grab it if you can..
6) Stay away from Sony...

Too many recall problems and overpriced to boot.. they look nice though.
7) Check out Dell/Acer websites.. you typically can find some discontinued models (3 months old) for a great deall.. and then you can apply 25% discounts and $30 off coupons to bring the price down even lower!
Ok.. that's it from me... good luck and let me know if you need help.