As pointed out by 2hvy4grvty, basically,
L1 Cache > L2 Cache > L3 Cache (Now called Last Level Cache, LLC, by Intel) > RAM (In general here) > SSD > HDD. Of course, you can't get more cache since its built into the processor, but if you are wanting to speed up your computer, changing to a SSD might yield more noticeable day-to-day performance than upgrading RAM.
This is because, if you get more RAM, you'll only notice it in cases where you have many windows open / heavy multitasking. Sure, I do virtual machining, so 8GB is certainly noticeable once I get my Windows 7 VM open, but if you're just doing some basic work, not a big chunk of difference.
A SSD is different though. It will make opening apps, saving things, reading things, much faster than a normal hard drive.
Swap files on SSDs won't replace RAM anytime soon. SSD read/write speeds peak out at ~500MB/s, while RAM could hit ~10GB/s. SSD speeds might go higher, but well, RAM will go higher, too. We won't see a merge anytime soon.
Perhaps we'll see RAM and SSDs merge when Mac and Windows merges. /s
