If you are sure you can sell your current CPU, then for sure I'd probably do it because overclocking fun is worth more than $30 to me
Besides you can probably get that boy to 2.5-2.6ghz if you try hard. Performance wise, it might not be worth it and since you already have a decent setup why the hassle? But if you enjoy overclocking and don't mind spending $30, I'd do it.
On the other hand, you can already get 6600GT AGP. Sure it costs $225 on Pricewatch vs. $180 for PCIe version. However, think of it like this: $225 - $30 you'd spend to get the mobile, so your cost is now $195. So get the 6600GT AGP now and enjoy playing games on it now, not 2 months from now when the price falls to $180 for the AGP version.