Well you have a couple different issues.
1. I buy a core duo right now, how much to upgrade it later.
The core duo right now is in no means a slouch. If you bought a 2.0 T-2500 it isn't like you are going to say, "Oh, this thing is slow, I am just getting by." On the contrary you are probably going to be like me and say, "Holy crap, this is insane for a notebook compared to a year ago!" I honestly don't think when the core-duo 2 comes out you are going to say, "I have to have this (at these prices).
What I am saying is wait until winter, spring, whatever (basically until the price is way down) buy a CPU to upgrade after Conroe, Mermaid yada, yada, yada is out and Intel and AMD beat the snot out of each other on pricing, get yourself a 2.9 Ghz core duo, add another 1.8 Ghz of power to your machine, almost the equivlent of an entire CORE of the biggest Turion X2 at this point. Look on Ebay right now for Pentium M's... they're free, I spend more on a tank of gas than a 2.0 Ghz Pentium M. Same thing is going to happen to the core duo 2. What will it cost to upgrad it? Depends on how soon you have to have it.
2. You asked about gaming between a core duo and a turion X2.
Supposedly AMD is going to be better, but I can't look at any benchmarks for systems that don't even exist. Then again, it isn't going to be better than a core duo 2, and AMD has already admitted that in saying they are going to put dual dualcore cpu's on a single motherboard in the desktop environment to "retain the performance crown", I already said this in another post, but you don't "retain the performance crown" by pitting 2 processors against one... that's idiotic! Do they think Intel is just going to sit there and say, "Oh Cheese! how will we ever come up with a way to counter that! It is going to take so much freaking intelligence to put a second processor on a motherboard like AMD did... I don't think we can do it."
My next question is: What is the Turion X2 going to cost? AMD is kind of famous for charging too much for their products. Look at a Turion MT-40 $220 compared to a Core Duo 1.86 at the same price. Look at the desktop market, same thing... overpriced. Even when the X2's come out they, I am assuming here, will be priced equivalently to the core duo 2.
I also wonder on the heat issue with the Turion X2. I can tell you a core duo has WAY less heat than a Turion single core, and I have 2 units with 6 cell batteries. The Turion lasts an hour and a half if you're lucky and hte Intel lasts 2 1/2.
Finally figure in that AMD tops out at 2.0 Ghz for the dual core. The current Intel core duo runs at 2.33 Ghz and the next generation is already up to 2.9 Ghz. So even if the core duo (or core duo 2) is (for the sake of argument) the exact same gaming power output clock for clock Intel already has 330 Mhz x 2 cores (for the core duo) and 900 mhx x 2 cores for the core duo 2) of firepower they can bring to any battle.
I think that the Intel machines are better quality myself, after seeing them both inside and out.