We can't decide on clocks and stream processor in these two cards. What we can decide is by performance, how? By looking at the reviews. In your case the 440 will mop the 6450. To be honest I have no Idea why you are comparing these two cards, because they are totally from different levels. The 6450 is priced below $50 and the 440 is about $75. If you want something more comparable to the 440 I suggest the 6670.
HD6870 900/1050mhz 1120 Steam Processors
The 6870 is faster than the stock 5850, but the 5870 is faster than the 6870. Clock speed is nothing, same with Steam Processors, UNLESS they are the from the same series like the 58xx, they usually have the same chip and are made the same.