Unstable overclock even at 43 degree

Increase voltages in 0.025 increments. Each chip has its own particular profile, so you can't pay too much attention to other's results, however I've listed the following if you're interested.

Ian Cutress from Anandtech used to be a hardcore overclocker (perhaps Pro, or at least active in the high level amateur scene), and I like the way he lays out his results. His review of your CPU is here: http://www.anandtech.com/show/8232/overclockable-pentium-anniversary-edition-review-the-intel-pentium-g3258-ae
The results I'm talking about are in a table a little past half way through the page.
You can see he's been able to undervolt his CPU to 0.925 volts at 3.5Ghz, and then required gradual and usually small increases in volts from there until about 4Ghz, after which each 100mhz step required more significant voltage increases until at 4.8Ghz, even 1.525volts results in a BSOD (1.525v being way higher than most would recommend you run at).

He got a review sample from Intel, so you'd have to suspect they cherry-picked him a good CPU. Don't expect your CPU to be as good and if it is... you're lucky!

It sounds like you've got to 3.9Ghz just fine. Why not start yourself a table like that page has, and see how far you can get. Set yourself a voltage and temp limit, many suggest that you don't go over 80 degress on a stress test and 1.4V as the max... as soon as you exceed either of those, dial back to your previous "successful" row and you have your maximum OC.
With that table as well, you can then look a few rows up and see whether the last few hundred Mhz are worth it. In that Anandtech table, for example, The jump from 4.5 to 4.7 came at the cost of 0.125V (5 steps) and 12 degrees. Given that voltage and temps both affect the life of the CPU, if I'm wanting to get 4+ years out of my rig I might choose to dial back to 4.5Ghz, because the extra 4% or so performance doesn't justify the extra stress. On the other hand, if the Pentium is just a stop gap until I could afford an i5 or i7, I might be happy to push it even hard (perhaps past 1.4V even), to squeeze out every bit I can because long-life doesn't matter to me.
The point is, with a table like that you get a sense of how your particular CPU OCs, and where you want to leave things long term.

RE RAM, there are very few applications that benefit from increased RAM throughput, and Intel's memory controller is already very strong. Generally faster RAM or overclocking RAM is more of hobby than a way to get better performance, unless you happen to rely on one of the few situations where RAM speed is holding your system back (gaming with an onboard GPU is one such example).