How long before this computer becomes obsolete?

That will support gaming for a long time...look at it this way. Right now people run games efficiently with Core 2 Duos.... a Core i7 is an enthusiast CPU. It will be great. Maybe someday you may get another 4870 and CF them, but that setup will be fine. You can also increase the RAM as you need to.

Depends what performance you want out of it. The card will be "obsolete" first, then you might want some more RAM later.

You should also go with a triple channel RAM kit for a little more performance.

It depends on you. My gaming computer plays rts games such as Red Alert 3 on ancient hardware at 1680 x 1050 very well. Just an ancient Athlon XP and an HD 3850.

They are working on its replacement as we speak.

This. Gordon Moore (and others) have understood this for over 100 years.

Find the sweet spot where you get 85-90% of the performance for 50% of the cost.

Obsolete is a silly word. There will be better parts for every aspect of your rig in a month.

How long will you get to game out of it? Well that is up to you. My brother games on an old althon x2, with a radeon 2600.. plays WOW well enough for him.. I go nuts not being able to play at max settings at 1900*1200 at all times. He upgrades every 3 years.. I tend to always be upgrading something..

That rig will last you a long time though, no doubt. How long is entirely up to you.. I bet it will still play games at medium settings for 4 years... May not be able to keep up on high settings as soon as dx11 comes out.. who knows.

And my airplane starts to run out of fuel the minute they take off