I pretty much make a major upgrade every year during the holidays (between Christmas and New Years). I'm glad that I've got the time to wait because there is so much that is unknown right now, or that is just not quite right, or just simply not necessary (for me).
There is no question that C2D is faster than X2. However, if you're focusing on games, unless you're running at low resolutions, or disabling a lot of the eye-candy, then you won't notice any difference between a C2D and X2 system. I know that I could upgrade to C2D, but it would absolutely not improve my framerates in any noticeable way based upon my preferred video settings. I like to run with in-game features set pretty high, and with 4xAA, and with 8x (or higher) AF. I use a single 1900XT 512MB card. As it is, I have been eyeing up a bigger monitor (which means higher res), and a faster card or SLI/CF configuration would be what I need more than a faster processor.
Regardless, I think the platforms available for C2D leave a lot to be desired, especially CF/SLI boards. 975X boards are fairly pricey. 965 boards don't even offer 2 x X8 PCIe slots (1 x X16 and 1 x X4). Nvidia is about to release a new chipset, so I'd be hesitant to jump on 5xx-series boards right now.
Memory prices are quite high at the moment, and it would cost a fair chunk to buy CPU, motherboard, and memory.
Finally, there is always the big K8L question mark. Everyone has their own opinion on it, but that's just it. They are opinions. Who would have believed that C2D could have seen the huge increases over CD? K8L might be a slight improvement (read "huge disappointment"), or it might be to K8 what C2D is to CD. Although K8L is still quite a ways away from shipping, remember that Intel demoed C2D several months before it launched. It's possible AMD may pull the same stunt and be demonstrating K8L sooner than people think.
If you really want to go dual core, take the easiest path for now. X2 is a great choice, and won't set you back too much. It will definitely buy you some time, and let you see how things shake out over the next 6 to 8 months.