Without knowing the specs of your previous laptops, or how many and which programs you would run at one time, its impossible to give any kind of comarison
But seriously, im amazed at what the ibook can handle. Just today out of nothing more then sheer boredom, i opened every program in my dock to see if it would freeze. Thats photoshop, the whole ilife suite, google earth, safari, mail, transmit and some others, and it didnt freeze! Even exposee didnt freeze it (looked cool with all those windows shrinking though.. lol). This is on the 12'' with the default 512mb ram, i imagine 1.5gb would improve it drastically still.
But if i tried that on any of my old windows laptops i know it would freeze.
I imagine if money was no object to you then you wouldnt be here asking, you would just buy the best and most expensive laptop out there. But I think the iBook is the best value for the money.