I agree with wildman2 on a few things. If I had the money, I would drop the 8800GTX video card too(don't need that power for 3D apps unless your doing mega high poly counts in viewports and shading) , and get a better processor or monitor, unless you game ALOT. And agree that a heavy duty power supply will be good also. Windows x64 Pro OS, unless Vista has something you want. 2 gig ram should do for now, but make sure board can support better/more etc... 

edit: as for external HDD, that will work for saving things, but you can't go wrong with a good DVD or better disc backup of projects, cause HDD's can still loose data very fast.