First and foremost, what do you want to do with your system, because I’ve been multitasking for years with a similar system (the processor was 2.0 ghz, dual core) and it ran just about everything including Crysis 2 (barely) fairly well (I upgraded about 2 months ago… finally).
Second, is it a Laptop or Desktop? Because with a laptop, you’re better off buying a new one, those mother boards are made for specific parts, and they’re not as flexible as desktops in terms of parts (not saying it can’t be done, but it’s a major pain in the ass).
What in the hell is your Mother Board… before you upgrade ANYTHING on your system, get the name, google it, and check the maximum RAM, CPU speed, etc. it can handle. If you don’t, at best, you’ll prevent it from starting up, at worst, you’ll cause your system go into a nuclear melt-down and then spend about another $1000 USD (convert currency here) on new parts… I’m not kidding.
In your position, I would check your mobo (mother board) first, then go from there. If you don’t want your system so bogged down, try running a virus scan, spy/mal-ware removal, and anything to cleanse your system. If worst comes to worst, just do a wipe and reinstall windows 7.