Request Custom formating

I am having a custom formatting expression in Excel 2003 which displays "-" if the cell value is 0.
The expression I am using is the following.

_(* #,##0_);_(* (#,##0);_(* "-"??_);_(@_)

I need to change this for the following conditions
If cell value >500 then should show blank and if cell value =0 then should display "-". I am new to excel custom formating.
Appreciate the help from experts at the earliest for this.

Thanking you in Advance,
Raj