You'll need to use VBA to compare the cell value to another cell everytime the worksheet updates. I assume you're using a DDE feed or something to get the data in? At any rate, it's not hard to do, the basic logic is this.
Assuming the value is in A1 and you want the high in B1
if A1 > B1 then
B1 = A1
end if
The only thing to remember is that if B1 is blank, it will be treated as 0.
Hope that helps,
- The New Guy