AMD Athlon™ II X3 425 (2.7GHz, 1.5MB) how is it compared to Pent 3.40GHz?

For this I'm assuming that you're talking about the Intel Pentium 4 Extreme Edition 3.40 gHz...
Don't let the higher processor speed fool you. Just because 3.4 gHz is a higher number than 2.7 gHz, it does not mean a faster computer. A real component affecting speed is the Front Side Bus speed [FSB]. This is how fast data can be transferred to the rest of the components, and is usually not even thought about by consumers.

The AMD Athlon™ II X3 425 has a 667 mHz memory controller and a 2000 MHz 16-bit HyperTransport link.

The Pentium has a 800 mHz FSB.

This is the best information I could find about this crucial comparison, and it's not very clear.
The AMD has 3 times as much L2 cache while the Intel seems to have faster technology accessing it.
Another major difference is that the AMD is 64 bit while the Intel remains 32 bit.

This information was found at this website under each processor talked about here:
http://www.cpu-world.com

Now for real testing, and probably more accurate information:
AMD: http://www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+Athlon+II+X3+425
Intel: http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+3.40GHz

Scroll down to see the CPU in brackets. The AMD blew the Intel out of the water.