I'll echo many here and say that once you get SSDs, you won't go back... especially on a laptop or other "quieter" system (the difference is pretty impressive). If you've got 5x 120mm YS Tech screamers, you're not going to hear a difference, but it's creepy how quiet my laptop is now.
The whole idea behind making your system faster is determining your needs and then ensuring that you can get the data to the processor fast enough to keep it busy. I went with 12gb of ram on the desktop because I needed to work with huge data sets, but I also went with a pair of SSD's in R0 because I like being able to load programs and use the computer at a rate to which I've become accustomed. Keeping my programs microseconds away from the processing units makes my computing experience feel much smoother.
That being said, mass storage on SSDs is totally impractical. The price/size ratio is just too high to support storing your porn on an SSD (and if you can't wait that long, then it's time to get a gf, lol). For things like this I would suggest a decent array of spindle disks (i've got 8x WD Green TB's in R5 for movies, tv, and "other stuff"). Even on an array and with fairly slow HDDs, this stuff streams HD just fine.
Long story short, it comes down to knowing the data sizes you're going to use with the computer, the frequency they will have to be fetched, and trying to echelon your storage to ensure that the processor is kept busy to the highest degree possible.