The X2 3800 is easily the best buy around for a lot of people when most of us cannot utilize the faster dual cores well in our typical use. Not many people do much video encoding for example.
Today the AM2 X2 3800 is $135 at Newegg.
To offer the same price/performance ratio for example, the intel e6400 would have to be near $180 (instead of it's actual $223).
But buy what you need, and if you don't need super fast cpus (few actually can utilize the speed), then buy what is efficient for your dollars.
Even with extremely heavy multitasking (7 kinds of software at once, 30 windows) with modern security software, etc., my X2 4200 is almost never over 70% utilization more than 1 second at a time, and never is the slow part of my system.
Instead of paying $80 or $150 more for a lot of extra cpu speed we don't use, for most of us the better choice would be a AM2 X2 3800 now, and upgrade later again for a modest price in a year, with more money available now for DDR2 800 or a better hard drive, or another step up in the graphics.
To balance things for your preferences, see "Rules of Thumb" here, and "What is a Basic ...."
by that logic the e6400 is better than your e6300.
By total cost and price/performance, the AM2 X2 3800 is somewhat better, since it has the best price/performance of the 3, the lowest price, and is adequate for most uses that most people do right now.