Is it just a waste of money to get 16GB and instead get a SSD drive? Or is 8GB the optimum?
Some people say that if people have only 4GB in their 64bit system, only 2GB is usable and the rest 2 is already allocated to the system.
You can think of it as such that with a 64bit system, only 2GB is 'useable' of the rest while 2GB are already allocated to the system I suppose! But your systems already needs 2! The other way to think of it is that a 64bit system requires 2GB for the system itself but the rest after those 2 are availible to be allocated to be used for everything else... but how much is the optimum max, 6, 8, 12, 16, 32?
The only issue for you is weither 8GB or 16GB of RAM is worth it for you or if the extra price difference could be better put towards something like an SSD especially in relation to any different improvements each option can be gained. But even DDR3 RAM is much cheaper now days while an SSD is still quite expensive, but their price has also significantly come down too and their size has increased. (I just bought a 256GB SSD for just under a dollar per GB, finally!!!)
I'm no expert but... I have also read quite a bit on the question of how much RAM, 8GB or 16GB and TBH, my 'understanding' is that 8GB is mostly all you'll use at most for most things AIUI and there is any benefit with double that amount, unless Windows 8 changes the paradigm. Therefore 8GB probably would be optimum especially if it allows you to afford an SSD, especially since the performance improvements from an upgrade to a sufficient sized SSD for at least a boot drive will far outweigh the extra RAM, which m ight very well be a waste anyway. (An SSD might even be a better benefit just for using to place the Page file aka swap file on too, than having the extra RAM, quite frankly.)
Also, bear in mind that the speed of the RAM is just as important as the size of it, I'm using a fast set low latency 2GB in a 32bit system instead of a slower higher latency set of 4GB that I've tried. So you might be better off paying a little more for a set of low latency 1333 or 1600 at least 8GB instead of a cheap set of high latency 16GB at 1066 while getting a boot sized SSD if affordable on your budget, even if you were only going to use it for the Page or swap file.
16GB is a bit of marketing hype by the RAM manufacturers to a degree as is >2GB GPU RAM by Graphics card makers unless you have 3 32inch displays...