That option is only there for laptops. You can force max. clocks with 3rd party software (maybe Afterburner, Radeonpro, ATT etc.) or by editing the BIOS.
However, think about it: is it really worth it? Yes, you'll have 342 fps instead of 339, your epeen gets larger. Still, there's more power consumption, more heat, risk of bricking the card etc. The card downclocks itself because of
2. idleing (slow CPU, crappy console port etc.)
3. outdated bios (ask the manufacturer)
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64