Sorry - I meant "continuous".

tg

Every release.

No.

System requirements don't increase every release, you should be able to keep a machine for 2-5 years. It also depends on what you model, some people do work that would run on bare system requirements, other require more or faster hardware.

With engineering software there is revisions to codes which need to be updated regularly. There is also the need for continual improvement and making software more user friendly to stay competitive. For cad there are new features and tools, some like multi-bodies, frame generator, pattern of a pattern, etc are all well worth the upgrade. Don't forget if you share files with contractors the software is not backwards compatible.

I have an i7 with 8GB of RAM and an FX1800 running windows 7 x64. I have had it for 2 years and works fine with relatively complex and large number of component files. It will last me probably another 2 before I will need (or more likely want) to upgrade. However for the FEA work we do I may need to upgrade sooner, or just the RAM at least.

I would rather the cash go into my pocket than the office hardware but if funding isn't a problem upgrade your PC as often as you like.

We upgrade every year..(I actually haven't installed 2012 yet though because I'm too busy).
My hardware like said above will last 3-5 years typically. Only thing I might do is add a solid state hard drive when prices come down a little.

If you are happy with what you have stick with it. I always find something in a new release though that makes me want to switch. "oh neat.. I can do this now,etc.."
 

Also, Inventor has an advantage in that it uses Direct3D Rendering.  This means inexpensive (relatively) gaming graphics cards are actually superior to the absurdly priced workstation (Quadro) GFX cards.  
So even if you have to upgrade, you can build an entirely new system for less than the subscription cost of one seat of Inventor.