For most people, I would say not to get a flash drive until the memory has been maxed out. At that point, a flash drive will probably help more than the price difference on a faster CPU.
That's excellent advice. Its pointless to spend money on an expensive disk before you max out your RAM. Unless the OS is really brain dead, it will cache reads fairly frequently and that is still much faster than any SSD can be.
Most of the time, that is. There are situations when you'll have write-heavy workloads or a dataset that's larger than your RAM can be. But then I'd also assume you are out of the personal computer league anyway.
I would disagree that most people won't benefit from an SSD. If I were building a computer, I think I might set aside money for one before even considering the other components. Thankfully, SSDs are now relatively cheap. It would cost me $200 to get 32GB of RAM. A good 60GB SSD can be had for around $75.
Fast writes do make a different for normal desktop workloads, and fast reads are noticeable despite OS caching. If nothing else, you always have to read something from disk at least once, and you always have to write your dirty pages to disk eventually. The difference becomes more noticeable as the contention for disk access increases. No matter what I'm doing on my computer with a SSD, I never feel like I have to wait. On my other computers, even browsing the web while performing an `apt-get upgrade` can feel unbearably slow.
60 GB is not that much storage space if you consider the amount of music and photos an average user can generate. My photos alone are over 20 GB right now.
Then there is the frequency with which you reboot your computer. It's true every item must be read at least once, but, with enough RAM, it's read only once per boot - if you keep your machine on for a month at at time, reads to /bin will hit the disk only once every month or so. Disk writes can be trickier, since waiting for the physical write may halt the thread for a while, but, unless you specify writes to be synchronous, there is no reason not to trust the OS with the data and let it flush the cache when it's more convenient. And subsequent reads to the data you wrote won't hit the disk again until the memory is needed for something else. Reads from RAM are still orders of magnitude faster than reads from disk.
I agree that once you have more RAM than your usual workload and the size of the most frequently used files, adding more memory will have little effect and, when you get there (say 8 GB or so) you are better off spending your money on a good SSD. Given the failure modes I keep reading about, I suggest getting a smalled SSD, large enough to fit your software, and a larger hard disk where you rsync it from time to time.
