KneeTheCap: Athlon II X4 is my current CPU, though I am in the process of upgrading that to either intel's i5 or i7 in the not-so-distant future. Yes, that ugrade will also include motherboard too.
If you have an AMD CPU now you have to upgrade your motherboard. :P
If you plan to get an i5 or i7 then you don't need to bother with the bottleneck part I talked about. Just buy one of the GPUs I suggested, the one
you think you can afford. (I highlighted you because I don't think your wife should had a say in this unless she's an economic or hardware expert ;)
I saw an earlier remark you mentioned about a GPU that will last a couple of years. I will say this about it: you can't know that in advance due to how technology changes but I would lean towards a
no depending on what your requirements are for performance and graphics in general. If you have high requirements of one or even both of them then it's an absolute no, even with a high-end GPU for $500.
I found some crude ideas on some hardware forums where people calculated the best approach towards upgrading your computer, like the famous buy a high end card and wait longer or buy a low to med end card but more often. What's wiser? I don't remember any numbers really but the gist of the summary was that it's ideal to buy a medium to med/high-end card if you care about performance / cost ratio (best bang for the buck) but more often unless you don't really need the latest tech and is willing to buy an even older card because you only play less demanding games (or you don't have high requirements).
If you like to take a scientific approach to see what your needs actually are I suggest you download a framerate application like FRAPS then do systematic tests between low, medium to high demanding games with different graphics and find out how important either framerates or graphical fidelity is to you. This is good because then you might notice information that may help you in your need when you upgrade. For example, I prefer 60FPS in games but I don't value AA, shadows or the relatively new setting Ambient Occlusion enough that I'm willing to sacrifice my performance. You on the other hand might value shadow quality for immersion but at the same time is fine with 40FPS.