Higher ISO also induces more noise in most of the camera's , and with the higher ISO you want a tripod ( which you should use anyway ). A good image always comes back to lighting , you need light , plain and simple. ( Bright direct sunlight is not flattering to knives at all ).
Using either the built in flash or a hot shoe on camera flash will not give you exceptional pictures. You can make a very simple set up very cheap , but you still need to understand what settings to use on the camera.
I recommend this book:
[ame="http://www.amazon.com/Understanding-Exposure-Photographs-Digital-Updated/dp/0817463003"]Amazon.com: Understanding Exposure: How to Shoot Great Photographs with a Film or Digital Camera (Updated Edition) (9780817463007): Bryan Peterson: Books[/ame]
Learn exposure and your pictures will get better very fast , as once you understand exposure , you will then know what light you need , and where that light should be.
What lens are you using ?