You might want to try increasing the width and adding a negative margin on hover, rather than using z-index.
Code:
#id {
width: 100px
}
#id:hover {
width: 110px;
margin-right: -10px
}
Obviously these dimensions are just examples, so replace with your own
The negative margin simply means the item to the right of the element is not 'pushed' over (the negative margin is equal to the additional width when hovered).