I would say you would have to upgrade your CPU (definitely) - but this also possibly upgrading the motherboard, if the mobo doesn't accept the new CPU or RAM you are going to put in.
Also, upgrade your graphics card and your RAM. Recommended to have 4 GB DDR3 RAM.
The system requirements (as you can read on this website) are very accurate.
You better have at least something in the likes of ATI HD5470 512 MB or Nvidia GeForce 9800 GT with a powerful CPU like i5 or i7 (like I have). I have tried this game on my brother's laptop which had a dualcore 2.3 GHz and 4 GB RAM and nope, the game ran extremely slow (4 FPS?) even on the lowest settings. You need a powerful machine for this game. Luckily I have one.
I run a mixture of high and medium (more medium than high) settings on this game.
Here are my system specs (my laptop):
Intel Core i5 480 2.66 GHz /core
4 GB DDR3 RAM
3 MB L2 cache
HDD 500 GB
ATI Radeon HD5470 512 MB
Edit: RAM is very important and I kind of changed my mind. Maybe a GeForce GTX260 would be nice (yes I know it is expensive).
'GTS' graphics cards are not good for gaming. GTX versions are more suitable and powerful.
This is your CPU. It is a 'high mid range CPU' and it classifies on the mid bottom of the list.
www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+Athlon+64+X2+Dual+Core+5200%2B
This is my CPU. It is a 'high end CPU', which means top of the line, though it is on the lower end of the list.
www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Core+i5+M+480+%40+2.67GHz
I recommend you get one of those high end CPU's.
It is not enough to tell us '8 GB RAM'. You have to tell us what type of RAM because it really matters (DDR3 is faster than DDR2 etc.).
Download 'Everest Home Edition' and run it and you will see what type of RAM you have when you navigate in it.
As for your graphics card I would say it's pretty good, so that is not where the problem lies. The problem is that your CPU has to display and handle a gazillion events on screen (cars moving, meters, physics - especially physics! rotation.. etc. in other words, a lot of heavy calculations. Your CPU is the big man here. So, get a better CPU).
Now all that remains is you telling us your RAM type/speed.
Thanks