At this point, RAM is in my opinion one of the larger potential bottlenecks for web apps.
Browsers can eat up a lot of memory while running JS-heavy "web apps", and leaks can result in hundreds of MB being used over time, causing excessive disk swapping etc.
On a laptop with WinXP and 512 MB of RAM, even Firefox using 200 MB of RAM (not an unrealistic number if you have a few tabs with mail, maps and other big sites open,) can start to hurt the overall system performance.
Real Users never know what they want, but they always know when your program doesn't deliver it.