Dual core cpus are much faster for multitasking than single core.
I don't know where you got this information about maximum addressable memory and the need for more than 4gigs of memory for dual core to really be useful.
A 64bit cpu can address waaaaay more than 128gigs of ram.
Addressable memory = 2^bit of the cpu.
2^32 = 4 gigabytes or 4,294,967,296 bits
2^64 = 18,446,744,073,709,551,616 bits
which is way more than 128 gigs.
Actually not really anything you said in your post is true....
1 core will be maxed by a CPU intensive program. With a dual-core cpu, that program can max out one core while the other continues to work on other tasks(applications).