Well, two player ( races ) or all of them don't change a lot since sins load ( cache ) everything at the start... same race not used in the play... very few files, mainly brush, are "load on demand"...
For speed up everything, you can edit your user.setting , located at :
C:\Documents and Settings\"your name"\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire\Setting
check the following :
ShowAsteroids FALSE ( dissable the extra asteroid in the gravity whell, not the one with resources max 10000 poly spared)
ShowElevators FALSE ( dissable all the object moving around the planet, become numerous with a full developped planet )
ShowDustClouds FALSE ( dissable the million of fine particule flooting in space max 5000 poly spared)
ShowPlanetRings FALSE ( bye bye saturn type planet )
ShowDebris FALSE ( dissable all the junk flooting in the gravity wheel )
dissable bloom... dissable AA... dissable the shake camera... use colored skybox or deepspace skybox, no both...
be sure to have :
IsTextureBackgroundLoadingOn TRUE ( use second core for load texture )
If during play, particle effect dissappear in the same file, increase the particle count... by example : MaxParticleCount 16000
At the OS level, stop process that you don't use like windows update, fast starter icon on the lower right ( icq, MSM, skype, openoffice, etc )... if single player, disconnet from the internet, stop firewall and antivirus... fine tuning your OS is the first step... i have XP pro x64 and at the start, only 130 mb ram is used... why have the sql server using 50 mb of ram when i don't need it for these game session... or steam using between 30 mb and 100mb... if i need one of them, i click a icon on my desktop but they are not enable on the startup... all memory and resource spared can be used by your game...
Take a look at :
http://www.optimizingpc.com/ ( very good one )