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.
Luck, that's when preparation and opportunity meet. -- P.E. Trudeau