Per GB prices of rapid SSD (like Intel X25-E) come close to prices of high-end RAM. So what extra advantages does SSD give you? What are particular reasons, why you buy SSD, instead of just putting more RAM in your server machine, to have it used as HDD cache or even create RAM-disk?
EDIT: of course I'm aware, that SSD are persistent. But so is the data in disk cache. Reading from RAM has got to be a lot faster, then reading from SSD. Also, SSD have slow write times, so no advantage over HDD there. Especially for sequential writes.
EDIT2: amount of RAM you can put is not so limited. With introduction of DDR3, it's not multiple of 2 anymore, it's multiple of 3. Standard SOHO MoBos have 6 slots, while server boards have 12 or even impressive 18 slots, supporting total of 144GB of RAM. Even if you use more cost effective 4GB memory sticks, you still can have 72GB.