8 GB is not to small for 99% of users and to big for 95% of users. If you use RAM drive then 16 GB is good but SSD technology is limiting the applications for RAM drive. I have only ever in my life seen the odd Video or Photo editor push past the need for 4 GB of RAM let alone 8 GB. Unless you have a 64 bit rendering application on the go then 4 GB should more than ample.
You are better off getting a faster single GPU set up. SLI can cause a lot of issues and its real world application is very limited. Not to mention the extra heat and the stronger load on the PSU.