We all know that a fast storage device helps with gaming, and that ram also helps. BUT
"if I had say 64 GB of ram (ie more than the size of the game on my hdd) and a slowish hdd for gaming, will the system load up files into the ram so that I have performance equal to or better than an SSD? Or will it only load as needed and hence the ssd with less ram would perform significantly better?"
no as RAM will hold the apps open where a hard drive wont it only reads and writes to the app.
for example having a 100,000 RPM HDD (not possible way to fast haha) and only 1GB RAM this will mean the app will load really really fast, but you wont be able to have many apps open at once but for a better gaming experience have loads of RAM and an SSD for best performance hope this helped.!
A HDD will bottleneck a new system with 8GB of RAM. 1G of RAM will bottleneck a new system with HDD.
It is all about getting the right balance. If the CPU needs the information, it asks the RAM for the information. If the RAM does not have it, then the RAM asks the HDD. With a SSD you cut down the times that it takes to get information that is not in the RAM. It makes your entire system seem snappier.
I would not bother getting more than 8GB of RAM for a gaming machine.
Ignore RAM drives, despite the awesomeness of one, I doubt you will ever use one.