I’m still not certain that the speed vs. cost factor leans all that greatly towards an SSD instead of more memory. Perhaps when they start matching prices with standard HDDs then I might consider purchasing one.
It's easier to justify on a laptop, where you also get to factor in increased battery life because you're not having to run spindle motors.
My opinion is RAM first SSD next. RAM will make everything work better and you won’t rely on your hard drive for swapping.
Once you quit swapping say 16GB, then an SSD makes sense because programs load and boots happen so much faster. You don’t want to use an SSD for Virtual Memory swap space. It’s faster but SSD’s have other issues. RAM is so cheap that it’s you big hitter day in and day out.