Whatever is on the motherboard is what you (apparently) have now. RealTek is an on the motherboard sound chip. There are also others such as IDT. A SoundBlaster sound would be a separate card and not on the motherboard.
You didn't specify whether you have a factory built PC (e.g. Dell, Gateway, etc) or if its a home built. If its a factory built PC, the PC vendor is the place to look for the correct Windows 7, 64 bit sound driver. If it is a home built (or custom built) then the motherboard vendor's site is where to look for the correct sound driver.
However, with an 8600GT, that suggests its a relatively old hardware design that may have been upgraded from XP to Windows 7 and the proper Windows 7 64 bit drivers may not be available from the PC or motherboard vendor. I used an 8600GT in an old Gateway Windows XP desktop PC I had (its been gone for 6 years). If there is no Windows 7 (or Vista) driver available for your specific sound module then the best option is to install a separate sound card that is Windows 7 compatible (XP drivers are not compatible as Windows handles some of the sound functions differently than XP). That is the only way to get full sound functionality.