I also have one core cranking at 100% when sins runs, starts to lag very annoyingly later into large and huge maps.
Same problem here... always one core at 100%... process jumping from one core to the other after some time...
On very huge maps ( 500 or more planet ), memory problem... more of 2 Gb used...
People with Xp and 4 Gb can edit the boot.ini and add the "/3GB" switch... Win XP with 4 GB ( not really 4 Gb since some address range are used for material ) use 2 GB for application and 2GB for kernel... Unless you run a server, you don't need so much for the kernel... the switch allow 3 GB for application.
Sins don't work with the "/PAE" mode... PAE allow 36 bits memory address on 32 bits processor...
About my own material : supermicro motherboard with two Xeon 2.66Ghz quad core, 16 GB Ram ECC 1-4-8 bit 4 channel ( mounted in mirror mode, mean 8 GB can be used ), Nvidia 8800 GTX 768mb...
With these machine, Sins is already very slow with the huge map single player... don't shoot the creator of Sins... very few application are able to use multiple core... the only one i know and use is Poser 7 ( a 3D render software ) who can use from 1 to 4 core for the render... "/PAE" mode is not supported by a lot of application since only the professional version of XP have a kernel who accept it...
My method for a good result on so machine was to use linux 64 bit with VM ( virtual machine )... by example, i can assign 5 core to the VM... so the VM "think" that the speed of my processor is 13.3Ghz( 2.66*5 )... for the Ram, i assign 4 Gb... problem on Linux, only the Nvidia card have a good support for fast 3D...