Greetings!
As someone that has spent many, many hours putting together projects on an iBook 500 (I render using the render engine on an even slower Mac when I do projects on the Mac – a 233 machine that just ticks away for a few days), with lights, shadows, and the whole 9 yards. I guess it depends on how “excellent” your preview has to be. I have a custom setting I use (instead of 1/4, I tell it to use 1/8), leave all the layers set to draft, and use the shift-ram preview regularly to check animation speeds, and usually don’t turn on shadows, etc. until the last moment, and even then, I turn on the caps lock, and usually “spot check” specific areas in full res. If I want a “great” RAM preview, I hit the 0 key, and do something else for a while.
That said (for the lower end way to achieve workability), more RAM won’t hurt, for sure, because your OS is using up most of that. Frequently the problem with a PC however, is the other running processes, or often spyware on the machine. I’d make sure that it’s clean, then disk defrag, and consider making sure that a lot of background apps aren’t always running (usually, but not always visible on the lower right in the menu bar). These are all taking a small bit of CPU and memory.
Good luck!
Rhett