Even real time effects aren't typically memory intensive.
If you're not using the memory, then you're not increasing your "speed." The reason that memory upgrades can increase performance is that memory is considerably faster than the hard drive (8ms vs 8-9ns in access time alond). The more the system has to swap out to disk, the worse your performance.
Typically a DAW should need a maximum of 256MB for most audio work. If running Windows 98, 128MB is usually sufficient. On Windows 2000 I have rarely gone over 128MB during a recording session, and that leaves an additional 128MB for loading soundfonts or loops that will be triggered from memory.
I think it's sort of funny when I see people putting 512MB+ in recording machines when they're just working in n-Track or whatever. If you're not using it, you're losing money.
Typically signs of being short on memory are excessive disk thrashing, especially during simple tasks like minimizing and maximizing windows, etc. Of course your disk will thrash when you're recording, but if the disk continues to run heavily after you've STOPPED playing or recording and are just working with the application, then you've got memory issues.
Slackmaster 2000