Sorry semp I just have to correct bad information when I see it and Ripley is a good provider of just that! 
It's quite the other way around mr. Tri-sli
It's absolutely laughable to make statements like a SSD won't improve the computers performance. Any computer with a good ssd and sata3 will feel like it's turbocharged on daily operations. Everything I've said so far is a verifyable and 100% fact where Chalenges opinnions are just opinnions, he cannot provide _any_ backing for them.
Chalenge fails to see the correlation between the claimed need for defragmentation in order for AH to run properly and i/o speeds.
Let's look at the fundamentals (simplified):
A physical hard drive is based on spinning platters which contain the data. The data must be read by metal stick(s) that follows the platter(s) physically. Even though the stick is very light and is controlled by very strong magnets, it has serious limitations on moving from platter position to another. While moving it must also wait for the platter to spin to the correct position before read can start (which by the way is one reason why a faster rpm hard drive of the same capacity is usually the faster one, a 5400 rpm large drive can be faster than a 7200rpm smaller drive but that's a whole another theory lesson there).
This is where fragmentation comes in. The physical drive can only be efficient in reading data if the data being read is stored in a continuous row on the platter, enabling the drive to keep reading 1 line without moving the head. Unfortunately NTFS and FAT file systems are built in a way which makes it possible for files to be spilled all around the platters depending on the use of the drive when the data is being written.
This leads to file fragments all around the drive and is called fragmentation. The drive now has to move the reading head 1-100... times in order to read that single file. Since every head movement will 1) Stop the reading of data and 2) Take a few milliseconds of time, drive performance suffers noticeably and you hear that 'brrr clickety click' dance from your harddrive. The problem is extrapolated when multiple different processes require disk access at the same time. Now the read head has to perform a breakdance all over the platters, and each process waits for the others to finish seeking and reading data. This can be partly avoided by Chalenges advice of saving data on a separate hard drive so it can read parallel to the system drive without the interference of other data being read and seeked. With many small files the read head moving (seek time) is actually more than the actual time to read the data
So the action of a hdd requires massives amount of waiting to move between points at platters, searching data etc. This inherent fault is known to the manufacturers which is why modern hdds all have cache memories. The cache memory is a little bit like tiny ssd inside the hdd. The hdd stores the data that was read but not yet required by the system at that moment in hope that this data would be required next. If it does happen, cache is hit and the hdd can for a very brief moment 'burst' data at SSD equivalent speeds. But this burst can only cover 8-32 megabytes of data that happens to be cached, nothing else.
Now lets compare this to a SSD. SSD has no moving parts. It has no spinning platters, it has practically speaking no seek times because it's based on memory chips controlled by a controller chip. A SSD can read fragmented files just as efficiently as non-fragmented ones.
This combined to the fact that a SSD can produce 3-4 times higher sustained output to begin with, make SSD a superior choice for anyone who hates waiting while starting operations on the computer. Or for AH if it needs to read a texture mid-flight, the file access is going to take 100x less time using a SSD compared to a fragmented HDD.
Chalenge is right that once you've started some app once it resides in ram - that is untill ram is filled at which time the cache will be dumped. This however does NOT mean that your computer will run only from ram from that point on. You're free to experiment how long it will take for windows to crash while unplugging your drive whilst using some application
Windows will constantly write temporary files and apps will constantly access data that does not reside in ram in the program binary - like AH which is "started" but still loads textures on the fly when need be. And this is where a SSD continues to give speed benefits.
It won't affect game FPS generally speaking, other than minimizing the chance of microhiccups or stutters caused by i/o. The SSD however will generally speaking make the computer perform much much faster, starting from the moment of bootup. I/O is the base of everything that happens on the computer.
