If you are considering the 820D, try to get the 920D. The 920D is the 820D's replacement and it'll only be maybe $20 more and falling. The 920D has double the cache so it'll improve performance a bit especially in the bandwidth constrained dual core circumstance of the processor. The 920D is also made on the new 65nm process to control power and heat. SpeedStep isn't working on the 920D right now though, but it may be corrected by a BIOS update in the future. Whether it will or not, the 820D doesn't have the feature to begin with.
In any case, it depends on what you plan on doing. If you mainly game then the single core is a good idea. Otherwise, you may benefit from a dual core. It's also interesting that if your game supports dual core your benefits can be huge. In Quake 4 with SMP enabled the 820D is faster than the FX-57.
If he can spare a few more bucks I'd say the AMD X2 3800, the 900 series from Intel still uses to much power. As for his price range he cant go wrong with an 820, as long as he understands the power draw of Intels.
The X2 is only $30 more?
It performs at least 10% faster, so that would be great value from where I sit. That kind of money for a lot better performance, and a lot less heat, is a real steal. Get the X2.
If I were to get an X2 3800+ that would cost about $75 more than the San Diego, I do want a dual core CPU. Is the X2 3800+ really better then the D 920 because the X2 3800+ only has 2GHz and 2 x 512KB L2 Cache when the D 920 has 2.8GHz and 2 x 2MB L2 Cache.
I cant comment on Intel other than the numbers I've seen. The 920 does lower the power draw but not enough. The AMD X2 3800 is still the best entry level dual core chip, but as I said if the 820 or 920 is in your price range and you understand your power bill will be more then get it. The AMD X2 3800 will probably blow the doors off anything (except higher clocked X2s) other than an Emergency Edition P4 (Just my personal opinion).
Ok looks like ill try go with the X2 3800+, last question is does this CPU have the equivalent to Hyper threading technology? And I know I could just google it but what exactly does Hyper-Transport do?
At the moment a higher clocked Single core will beat a dual core for games. Personally I dont miss 6 frames per second and most game makers are writing for dual cores now so a dual core is more future proof.
EDIT: And once you see how responsive a dual core is you'll wonder how you ever got along with a single core. (just my personal opinion)
Id Agree, And The Funny hing Is The 3800+ X2 At Stock Core Speeds Still Outruns Even The Mid-End 9xxx Series Dual-Cores In Most Tests (Especially Gaming Tests) Its Ironic Considering The 1000MHz+ Differance In Clock Speeds, So Id Say Hed Get A Better Value From An X2 (And One More Thing To Consider, In General AMD Motherboards Are Cheaper Than Intel Ones So Even If He Goes AMD X2 And Spends A Few More $$, In Reality Hed Still Be Getting The Better Deal... Think About It)