Assuming your text blocks are different text objects and total is a numerice field, you can use a script in the total field calculate event.
FormCalc:
if ($ lt 500) then
textBlock1.presence = "visible"
textBlock2.presence = "hidden"
else
textBlock1.presence = "hidden"
textBlock2.presence = "visible"
endif
JavaScript:
textBlock1.presence = this.rawValue < 500 ? "visible" : "hidden";
textBlock2.presence = this.rawValue < 500 ? "hidden" : "visible
";