I would guess that 95% of people would benefit far greater by spending the money on an SSD. 4 GB of RAM is plenty for most users - sure there are a few who could use 8 GB of RAM. I would suspect that the SSD would speed up an average users normal experiences 4-fold for things like startup times, large files, multiple read and writes and etc. Actually an HDD is the biggest constraint and also greatest rate of failure of any computing component. The SSD is a huge improvement not in just speed but also reducing read/write failures and minimizing the constraint.
Anyone considering 8 GB of RAM now should already have an SSD. Otherwise buy SSD now and wait two years until you might actually utilize RAM and buy for less than $100 per 4 GB stick.