I guess that depends on how much you bought your Q9450 for. If you got it for cheap then I would say you could probably sell a decent Q9450/Mobo/Ram for about 250 at the moment. It would cost you about 300 to do a used i5 750 upgrade and about 350 for a i7 860. If you went i7 920, probably 400-450. Those are all Used prices for Mobo/Chip/Ram (all or portion thereof). So subtract what you could get for you 9450 set up and do the math. Your stuff will only lose its value more until it reaches a point where it will make good replacement parts and then increase a little in value.
That all being said, performance wise from a i5 750 perspective, your not very far behind in speed. In tech (well I believe the Turbo Boost is overhyped for real overclockers), but its nice for those novices that don't want to risk their chips. So for performance sake, don't upgrade unless you really need the horsepower of a i7 920. And I can't say I do anything that would require that much power. I'd hate to recommend upgradeing to something you can never fully utilize and most probably won't for years to come.
I did a upgrade from Q6600 to i5 750 for a 150 dollar price difference after I sold my old stuff. Performance wise, its about a 25-33% gain in speed.