Hypertransport -
Because the memory controller is on the CPU there is no front side bus. Hypertransport (HTT) is the communication between the CPU and the various buses etc.
As for 2Gb - Well to get the most out your socket 939 you need 2 sticks of memory so it runs in "Dual Channel" mode.
So if you only get 1Gb you need 2x512Mb sticks.
Later when you upgrade them it is not recommended to get another 2x512Mb and populate all 4 memory slots as you have to slow the memory down.
So by getting 2x1Gb sticks now you bypass the upgrade without the extra cost of the 2x512Mb.
Also with 2Gb it is possible to cache all the skins and AH2 itself in system memory.