You've already got half of it? Aww well, I'd already written this, so.... if it bursts your bubble, it's a lesson learned in patience.
To be fair, your current PC is MORE than enough. I'm not going to argue against my own setup, but if I was buying the RAM myself, I certainly wouldn't have bothered. 6GB would have done me easily. My CPU isn't any better than your CPU either - or my old Q6600 - unless you've a very specific use for it - frequent video editing in my case.
If you want to make a real difference to performance buy some decent SSDs instead. If you want quality buy a decent monitor (and keyboard and mouse if you don't have them already). After all, what you interact with day-to-day is far more important than a 10 FPS, most likely more than your monitor can show you anyway.
I'd say 6 months, depending on what you do. Let's face it, you've not kept your current PC for long... that's hardly an argument for you.
If you game, you're getting next-to-nothing over what you already had. Heavy CPU-limited work, then fair enough.
In summary, you're either swimming in it, or you're an idiot.