I have recently swapped my San Diego 3700 for an X2 4400 and I'm glad I did!
My main worry was getting the same game performance whilst reaping the benifits of Dual Core. However, I needn't have worried as the X2 overclocks to the same 250x11 as my 3700 did. In fact the 4400 will happily clock to well over 2800 but I like the stability and low Vcore of 1.375v? My only reservation is the slightly high temps which vary from 28 deg cold through 32 deg idle/general gentle app's to 48-52 in games and occationaly 59 or even 61 deg when full out benchmarking
So, if you're still teetering on the edge of upgrading to an X2, DO IT! You won't be sorry.
Oh, and I didn't realise how many games already use both cores