In 2007+ you can use:

=IFERROR(VLOOKUP(A1,Sheet1!$A$1:$B$800,2,0),"")

For compatibility with 2003 and earlier, use:

=IF(ISNA(VLOOKUP(A1,Sheet1!$A$1:$B$800,2,0)),"",VLOOKUP(A1,Sheet1!$A$1:$B$800,2,0))