EDIT: As kooowhip_m16 points out my initial statement is misleading. It should say the Nazi's motives were not just economic, but ideological as well. Also read their response for a outline of Germany's wider economic situation.

Since more or less the beginning of his political career Hitler framed the Nazi party as Germany's best defense again Communism or "Judeo-Bolshevism" as he often called it. If you couldn't tell from that name their was a racial component to this enmity as well, with the USSR being seen as a nation of inferior Slavic peoples controlled by party of Jews, with some other racial enemies like Romani thrown in for good measure. In the Nazi ideology conflict with the Soviet Union was inevitable, as a part of the grand racial conflict central to their worldview. Another important Nazi concept was the idea of "Lebensraum" or "living space": that Germany should expand it's borders eastward to settle the increasingly large and urban German populace into agricultural communities, making the country more self-sufficient. This plan called for the extermination or expulsion of much of native population of Eastern Europe to make way for German colonization. The non-aggression pact and trade agreements were simply a short term strategic that they never intended to keep, and probably expected Stalin to break eventually as well.

As for why he invaded in 1941, there are a few explanations. The U.S. hadn't yet entered the war in spring of '41, and the British were confined to the Isles, being in no position to launch an amphibious invasion for the foreseeable future, so western Europe was fairly secure for the time being. As previously mentioned Hitler didn't trust Stalin to stick with the non-aggression pact forever, and wanted to start the war with the element of surprise. And at the same time the USSR was getting stronger, as industrialization increased and the army slowly recovered from Stalin's purges, so Hitler may have figured that time was of the esssence.