I'd say they both matter, the speed and the amount, I'd say for 3d rending/modelling and all that fun stuff, the more memory the better, not so much how fast the memory is. (But the memory speed still plays a big part).
The reason is, is because when rendering/gaming, the speed at which your computer uses the memory to retrieve information doesn't really matter as the more memory you have, the more information can be transferred (even if it's at a slower speed because of the memory speed) so. Get the 16GB of ram if you wanted to, but in all honesty, 8GB of ram is enough for the things you'll want to do. Gaming on any game will never use more than 4GB of memory at the same given time, hell, probably not even more than 2GB. 3d rending and that stuff wouldn't even breach the total 8GB even.
Can try, I don't recommend mixing sets of DRAM, even the same exact model, it can and often is problematic, the larger sets cost a little more, but it's because time has gone into testing the sticks to ensure all play together, the forums are full of of folks who have mixed packages of DRAM and can't get them to play