If you want your graphics card to last a very long time gaming, the 8800GTX is the way to go, especially if you are thinking of upgrading to Vista for Direct X 10 in new games.
From the cards you mentioned though, in engine graphics in 3DSM won't change much, and the speed of the viewports probably wouldn't change from card to card (even in high poly modelling), and its all processor rendering, which brings me to my next point 2 paras below
But for games, yes.
Since you are a 3DSM user like me though, upgrading to Vista may not be a great idea, yet...
I would definetly decide on a new processor though. Go with an Intel Dual Core, or even an Intel Quad core for rendering purposes, honestly
I'd personally go for a Dual Core if I could afford it.
Cheaper, faster and more effective.
Everything else seems okay, but of course a new motherboard will be in order from what you would have got with the FX.