Fast Mode and Heavy Car mode
Choose your car, then immediately after you choose the car hold down Up-Triangle-X until the race starts. This cheat enables fast/turbo mode to enable this cheat, while you are driving hold up and your car will get a speed of burst for as long as you hold up. But you have to steer the car as well as hold up. You'll get it. Do the same thing except hold Left + Square + Circle. When you hit a car that has stopped or heading towards you it will blow up in the sky and come back down.
Submitted by James_B. 

Getting back on the road really quick
If you crash into the side of the road press select and you will appear back on the road. This cheat saves damage if you press it right after you crash. This cheat is also great on Hot Pursuit mode.
Submitted by James_B. 

Phantom Car, Hot Rod and Helicopter
Start at the start menu. Go down to game options. Then User Name, then type in FLASH for the phantom, then click on end, a box will come up saying cheat activated. Then type in HOTROD and Then WHIRLY. Then go to any mode and the cars will be after the McLaren F1. Note: Helicopter is only available in test drive mode.
Submitted by James_B. 

Go to: Need for Speed: High Stakes Info Page | nGaming Home Page.