For your uses, i would say a laptop is the way to go. However, laptops are harder to upgrade as their components are very specific, though i think more modern ones are better in this respect. One of the main concerns anyone buying a new computer should have is the longevity - upgrade potential - you dont wanna be forking out again in a few years.
Hard drive size depends entirely on your usage. I download heavily (legally
) so i have a 500gb drive with room to add more. I would suggest minimum would be 250gb.
RAM depends whether you want to use windows vista or windows xp. Windows vista is more RAM heavy but has a much improved interface, especially for pictures, files etc like you said
"Also want it to be fast in terms of using multi applications." - relates directly to RAM, so 2GB would put you in good stead, i imagine.
Theres some cheap core 2 duo systems out which would again enhance the computers ability to do things simultaneously, as well as put you in good stead for the future.
The fact that you may want to use photography packages makes me suggest using a mouse rather than a touchpad, hate touchpads!!