The 950 is much newer and better. Clock speed alone is meaningless. Newer gen CPUs are more efficient with their clock cycles, so even though they may appear "slower" they do more work with each cycle.
But that's only the tip of the story. Both of those CPUs use different sockets, so if you're going for one, does that mean you're looking for a new mboard as well? And if you're you're getting a new mboard, perhaps you're intending to build a whole new system? And if so, then you've got a lot more questions then just "which CPU is better."
The forum has a template when asking about new builds, so I'd look for that. If you give us what you're intending to use your computer for ( so here I'd assume gaming" ) what your budget is for parts, and what parts you already have, we can be of much more help.
The change from the Intel® Core™ 2 Duo (which includes the Intel Pentium® E5800) to the Intel Core™ processors (which includes the Intel Core i7-950) yeilded around a 15% to 20% performance improvement over a same type processor of the older generation. Now since you are also going from a dual core to a quad core processor that might also yeild better performance in a number of applications.
So go with the Intel Core i7-950 or even better yet get a get a 2nd generation Intel Core processor like the Intel Core i5-2500K.
The 1st generation Clarkdale Core i series CPUs are about 10% - 11% faster than the Core 2 Duo CPUs clock for clock; i.e. the Core i series can process a little more instructions per clock cycle.
Many popular games can make use of 3 core and a few an use 4 cores. Most games only use two cores currently, but the trend is moving towards more than two cores; especially for games with a pretty good development budget and geared towards "hardcore" gamers.
even better yet get a get a 2nd generation Intel Core processor like the Intel Core i5-2500K.
I definitely agree here. If you're considering a 950, you're likely going to need a new mboard. And if you're getting a new mboard, you might as well get a LGA1155 board for the i5. That CPU is $50 cheaper than the 950 and will outperform it in most cases.