A big difference? No.
That isn't to say it won't help or be useless. More RAM is always better, but it won't make a noticeable difference, at least not in terms you're thinking.
If cars do much for you, think of it terms of horsepower. You wouldn't notice much difference between say 400 and 500 horsepower. Once you get to a certain point, it just doesn't make all that much difference.
Things change once Vista ships, or for that matter if you are using OS X. But with XP or anything before, it really won't be that much different.