Uegis, it doesn't...
The 3450 compares to the 2400pro
The 3470 compares to the 2400XT
The 3650 compares to the 2600pro
The 3670 compares to the 2600XT
The only difference is the 34xx and 36xx cards are fabbed on a smaller process, so they should theoretically be cheaper to produce and generate less heat at the same speed. Also, this new generation of cards have an improved UVD engine that should allow even the slowest card of the bunch (the 3450) to play back 1080p smoothly, which evidently was sometimes a problem with the 24xx series. At least this is the line that AMD marketing has been touting...
There is no reason I can see to get a 2600XT over a 3650 (just the opposite). If there were a significant price difference (i.e. the 2600XT was a LOT cheaper) then maybe. Inexplicably, however, most flavors of the 2600XT seem to still be MORE expensive than the $99 3650, which may be vaguely understandable for the DDR4 versions, but not the DDR3 versions. And frankly, if you are looking at the DDR4 version of the 2600XT, you'd be downright silly to not spend a couple extra dollars and get a 3850 instead. Bottom line, I would not get the 2600XT.