If you want my opinion, then NEITHER! First off, screw single-core. Too slow for intensive multitasking. Now for fual core. If you're looking at a 3800+ X2, that has 2x512mb L2 cache, totaling in 1mb, the same as most single-core processors. On newegg, the 3800+ cost around $285, so that's not that bad. In my opinion, get the Opteron 165. The Opteron is dual-core and has 2x1mb L2 cache, the same as the x2 4400+, but cheaper. This retails for $325, which in my opinion is a VERY good deal. Although it does come stock as 1.8ghZ (the 3800+ has 2.0ghZ), it can be insanely overclocked; I currently have mine running at 2.93ghZ with a Thermaltake big typhoon, faster than the FX60, which costs over $1000. The Opterons run much faster, overclock easier, and are cheaper. Overall a great buy. So I'd choose the Opteron 165 over the X2 3800+. Here's the link if you are interested in buying.
For someone with an average rig im really not so sure dual core is that much better..... yet. The people that ask these questions about entry level dual cores vs single cores arent going to have multiple hard-drives and wads of ram to allow for "real" multitasking. Decoding while playing games, maybe, if you want loading times to take an hour. Nope for regular joes, like me, that $100 (canadian) between the two should go towards upgrading the graphics card to a 7900gt or springing for some real quality ram.
Yes without a doubt you would be better off with the 3800+ X2. Multicores are the future of computers. More and more apps, games, and OSs will take advantage of the technology. You will get more performance in somethings today with the 3700+ but you will get better performance for a longer period in the near future with the 3800+ X2.
Agreed, the Opteron 165 is a well powerful piece of Silicon, that has Extreme performance and Oc'ing capabilities. But I dont think You will need that powerful of a chip, but then again, I take power over money ne day :twisted: