E8400 has a serious clock speed and cache advantage. The extra cache can actually put it over the top in some applications just like the raw clock speed advantage can. And of course it can also be overclocked pretty much for free.
yes, the e8400 is not your "average C2D", it's pretty fast for when it was released...
if you compare the G1610 to what it actually replaced from that era, the gain would be significant (e3200?), but, you could overclock the e3200 to 4GHz and probably match the G1610!
still, if you look at the g2020, it's a 2.9GHz CPU, but with 3MB of l3 (1610 have 2MB) it still looked OK compared to the e8400 at 4GHz
E8400 was the equivalent in price to a core i5 like the 3330, 3350P, 4430...