8 GB should be enough for most of your projects, if not all of them.
I run with an ASIO Buffer Size of 128 when doing recording, and then when I move to mixing/mastering I bump that way up to 1024. This allows the much heavier processing demands of many effects to be handled, and during mixing I am no longer worried about trying to play along (latency does not matter once you finish tracking).
I am wondering if your issues are related to the particular effects plugins you have loaded into the projects giving you problems with crackles/dropouts.
You can very easily and quickly test this theory out. Load up one of these projects where you hear the crackles/dropouts, and hit the letter 'E' on your computer keyboard, then hit Play. 'E' will tell Sonar to toggle either off or on, the bypassing of all effects that are loaded into the project.
Sooooo, if your crackles and dropouts go away after you bypass all effects, then that tells you that one or more of your project's loaded effects are having bad hair days. (Hitting 'E' again will turn all effects processing back on, by the way, when you are done testing it out).
Some plugins are simply NOT intended to be loaded during recording, and are meant to be used only once you have moved on to mixing/mastering.
These kinds of effects would include those that use something called Look-Ahead processing, where they read the data ahead of what is actually playing, and they need a giant ASIO Buffer Size to do that properly, which then would make it nearly impossible to try to record at the same time, because of the large latency present. The other way an effect may cause recording issues is if it for some reason chews through giant bunches of CPU to do its thing. Convoluted reverbs are notorious for causing these kinds of issues. I believe Boost11 is another one, and there are a bunch of other ones.
So, IF your bypass effects test shows that loaded effects are biting you in the butt, and you still have recording to do, then you might consider either completing your recording with effects bypassed, OR you could go through the process of figuring out which effect(s) cause the crackles/dropouts, and swap them out for some less needy plugin until you finished the recording.
Once you ARE done with the recording process, and move on to mixing, I would suggest you then might want to bump up ASIO Buffer Size way up - like I said - I use 1024. Then you could load those more intense plugins and with the bigger buffer, things should then playback properly while you mix.
You will always need to consider switching between a smaller ASIO Buffer Size, like 128, for recording, and using little or at least less costly effects until ready for mixing, then bumping ASIO Buffer Size way back up when ready for the mixing process, and the more costly effects. It's just the way things work.
I HOPE that some of the above helps your situation out,
Bob Bone