Well you seem to have it figured out pretty well actually.
You could go for the dual core upgrade, and yes it will probably last you another year with your current config. But after that you'll probably have to upgrade again. Wether you prefer spending a bit of money now and some more later, or rather spend a lot of money now so you can use it for a couple of years is up to you.
I'm not very familiar with ATI video cards but I think that's a pretty fast one. Probably more up to date than the cpu anyway. As far as the motherboard goes, I have yet to see it make any impact on gaming performance. Most of the motherboard's tasks is pretty much supporting the other new hardware you get (especially cpu/ram). RAM is sufficient, 4GB will be enough for some years and I usually only upgrade RAM when I get a new motherboard anyway.
You could try an SSD but it will only be useful in some cases. It will load your game data faster but will probably not do much as far as framerates go. Also, the price is still pretty steap for the good SSDs (Vertex, X25). I do not recommend getting any other SSD as they still suffer from new technology faults and they are not optimized well.
I've heard good things about the Phenoms. I'm an intel guy myself (for now) so can't really say much about it, but from what I've heard you'll get the most bang for your buck at the moment with a Phenom. So it'll be a good upgrade for you. Don't forget to look at the future though, get a platform for which you can get a new upgrade later on. Also, buying a Crossfire motherboard can be useful because you could add a second videocard later for a nice graphic boost, without having to spend a couple hundred dollars for a faster videocard.
As far as overclocking goes, you could always try it. That depends on what you do with hardware once you upgrade. I tend to just let hardware collect dust after an upgrade, so then you might aswell just overclock it to see what you can get out of it. If you want to use the older hardware for a second PC, or if you want to sell it, then overclocking is a no-go.