The update won't really affect you because your usage seem very light.
i3, i5 and i7 are "new" processors from Intel but because they are still dual core like Core 2 Duos are, there is no huge bump in performance. You likely won't even notice any difference. You won't benefit from more speed if you don't use it. Software is still mainly single-threaded meaning that it cannot utilize more than one core so Core 2 Duo will be more than fine for you and will last years.
Your usage doesn't require much from the GPU so 9400M is fine. RAM you can always add but 4GB should be plenty.
USB 3.0 is simply faster than USB 2.0 is so file transfers wibyll take less time. However, USB 2.0 is still fine for most people because they don't do big file transfers.
And this quote is a prime example of why one shouldn't trust Internet forums alone to make a computer buying decision.
The desktop i7 and i5 processors (i.e. the ones used in the current iMacs) are quad-core processors. Other than clock speed, the difference between the i7 and i5 is that the i7 uses Hyperthreading, which makes the CPU appear to the OS as if it has 8 cores. In either case, the i7 and i5 are SUBSTANTIALLY faster than the Core 2 Duo processors. Not only do they have more processing cores (4 vs 2), but the Nehalem architecture upon which they're based is substantially more efficient than the Core 2 architecture. To say that you won't notice a difference in performance between the i5/i7 iMac and the Core 2 Duo iMac is nonsense. I've owned both and the difference is significant. In addition, the i5 and i7 processors will automatically overclock themselves if fewer than four cores are in use. For single threaded applications, the 2.8GHz i7 processor available in the BTO iMac can increase its speed to 3.46GHz.
See here for more info on Turbo Boost: http://www.intel.com/support/processors/sb/cs-029908.htm
The mobile i7 and i5 processors as used in the newest MacBook pros are indeed dual core parts, but those processors obviously aren't used in the iMac line.
It's almost a certainty that when the inevitable iMac refresh hits the current Core 2 Duo processors will be replaced by dual-core Nehalem chips (most likely the Intel i3). My recommendation would be to either wait for the upcoming iMac refresh if you're looking for a dual-core CPU or get the quad-core i5/i7 iMac now.