sausagerules1 :
When I buy a graphics card, I never want to get rid of it, so if I bought a gtx 480 and then 8 months later bout a 660Ti, then another year later buy the more expensive GPUs now, (that won't be that expensive then), if that makes sense. Basically I never want to get rid of a graphics card, no matter how good/bad it is.
But I guess that 3-way would be ok, because I don't think that I'll ever get 4 graphics cards. I sort of want to "future proof" my pc in this way.
A couple of flaws in your thinking.
First, you can only SLI identical cards, different cards don't work together, and even it they did, performance would be degraded to the lowest level card you had, so buying better cards over time and adding them to an SLI setup would be a huge waste of money because the performance of the setup would be degraded to match the slowest, oldest card.
Second, 2 cards in SLI will give you about a 80% increase in performance, 3 will add another 25% on average, and the 4th most of the time will render 15% or less improvement. Your best bet is to buy the best single card you can afford now, then in a year, when the price of that card drops, and you feel performance starting to slip, buy another one. That is how you budget tight money on SLI.
Buying 2 cheap cards and putting them into SLI will almost always cost more and perform worse than a single high end card, always. Don't let anyone tell you different, its a bad idea to buy cheap cards and SLI them, BAD IDEA. You will not be impressed if you do. Toms sometimes recommends
SLI in their best GPU for the money articles, and I do not agree with them that this is a good thing to do value wise, over time.
SLI is not a budget solution. If your budget is that tight, then you need to look a good, fast single card, and just be happy with that.