you would need the windows equivelent plugins (most developers make mac and pc versions of their effects).
as for performance benefits, cpu is most important (both speed and number of cpus/cores). ram can help, esspecially if it allows you to use more cores for rendering – the genereral rule is 2-4gb per core, so you can do the math for your system. with cs5, ae can use even more ram than that.
as far as upgrading the cpu, some macs do have socketed cpus that can be upgraded. you would need to search the web to see if yours is one of those, and the process involved to upgrade it. you might also weigh the option of selling your current system and getting a faster one.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW