You change the setting in your BIOS, sometimes under advanced chipset features. However, running AGP 8x will not make your ti4600 "run better". The performance increase is negligible.
I wasn't aware of a Shuttle NB60N. Doesn't DFI offer a similar board?
** BTW, the TI4600 is AGP 4x only. The TI4800/4800SE are AGP 8x