I dont think that you need upgrade your system to install Windows 7.
Im running it just now with my dual core and 4 GB of RAM.
This game need DX10, I forced the game to use DX9 in my system, to see if runs fast but contrary as how should be it is slower
I guess the DX9 mode is not optimized.
You will gain FPS just upgrading to Windows 7, and it will looks better.
Watch this thread for example :
http://forum.1cpublishing.eu/showthread.php?t=22346