Merom clocks from 1.66 to 2.33 GHz just like Yonah. At equivalent clocks, Merom has 20% increased performance over Yonah. To compare, a 1.83 GHz Merom, will be around the same as a 2.16 GHz Yonah.
Merom is also 64-bit, which mostly concludes in highar RAM support. 32-bit processors normally only support 4 GB RAM, yet many of Intel's newer 32-bit processors have for quite some time supported up to 64 GB. The PowerMac G5, which is a 64-bit machine, supports up to 4 TB (4000 GB) or RAM. Since there have only been made 2 GB RAM sticks, the max is so far 16 GB, since it has 8 RAM slots. The 64-bit feature won't make much difference IMO, since Apple won't make 4 GB an option right away, since a 2 GB RAM stick is nearly $3k, which is more than a 17" MacBook Pro.
Despite the higher speed and support for more RAM, Merom's clocked at 2, 2.16 and 2.33 GHz sport a 4 MB L2 cache, instaed of the 2 MB L2 cache of the Yonah. The 1.66 and 1.83 GHz Merom's still have 2 MB L2 cache though. The Meorm remains with a 667 MHz FSB like the Yonah.
Just because Intel releases a new chip, ad Apple releases a new MacBook Pro or MacBook, doesn't mean that yous will become slower. It'll be as speedy as you've ever wished for. Just because there's something new, doesn't mean your stuff is out of date. I'm planning on getting a new MB when the decessor of Santa Rosa is released. 3 generations ahead of my current 2 GHz Yonah.
You wan't be able to put a new processor in your MacBook (Pro) since the processors are soldered onto the motherboard. This is however possible in iMac's and MacMini's, and will most probably also be possible in the Intel PowerMac (Mac Pro).
Santa Rosa is the decessor to Merom. Very little is known yet about it, yet Intel has announced that it will sport a 800 MHz FSB.
And to your actual question, I believe Yonah will be obsolete 3-4 years from now. By then, most new software will run extremely slow on them.
Just a warning: Never describe an Apple product with the word "obsolete", or Steve the King will send his Ninja G6 after you!