Yes, ReadyBoost will help.
But I think an 8 GB card will likely be overkill - with ReadyBoost, more is not always better. ReadyBoost works as an optimization for your existing RAM and page file. You're not storing or caching more information anywhere, you're putting the same information in a faster location.
Your page file rarely grows as large as 8 GB. It does happen, but most of the time it's sitting much closer to 4 GB, and so I recommend looking for a faster 4 GB card. That will be a closer match for your actual use. Check reputable reviewers for actual benchmark data, as the faster the card, the more the speed boost.
Even better, most netbooks support up to 2 GB of real memory. ReadyBoost will help, but the 2 GB upgrade will help more.