sorry cazzz ... but that's absolute rubbish
1Gb is a HUGE amount of system ram.
Crysis, bioshock call of duty4 will all work very hapily in 1Gb
as would most graphics and movie editors.
To answer your question adam Yes you could update by adding 2 more 512 sticks ....
look on
http://crucial.com/
and run the online scanner
or here
http://www.kingston.com/
but I seriously doubt that you'd see any real difference to anything but your bank account.
the sims isn't a particularly memory hungry game ... but a simulator does need a lot of processor time.
If your machine is a couple of years old you could probably benefit more from a complete system rebuild (I do mine about every 12 months)
but by far the easiest thing to try first .....
Clear out some of the detritus by downloading CC Cleaner (don't get too carried away ... just temp files!)
and run the defragger.
.... you might be surprised