It depends really, some people buy SLi motherboards and 1 GPU, and then when the upgrading season comes, they buy a second card and add it in. There are lots of pro's and con's to SLi'ing, and as
Rastalovich stated, in the end, it all comes down to how much money you have to spare.
For example, one GTX260 will cost ~£200, and two 9800GTX+'s will cost ~£300. If you were to buy both of these and compare cost and performance in both instances, you'd discover that you're paying 50% more on the 9800GTX+'s than you are on the GTX260 for slightly worse performance.
In some cases, SLi can be beneficial. For example, if you were to pay someone £100 for one 8800GT, and then he said that he'd throw a second one in for £20 extra, you have to think about it theoretically. If adding a second 8800GT and SLi'ing them would offer a 40% increase in performance over the first card, and you're only paying £20 for it, that's 1% increase in performance for every 50p spent. If you think about it as a ratio, in terms of Money

erformance Increase then its much easier to speculate. On the first card you payed for, you got 1% performance for 100p (or £1), a 100:1 ratio. On the second card, you got 1% performance for 50p, a 50:1 ratio, making the second card much more affordable than the first. Hence, someone going SLi instead of buying a better primary card and chucking out the old one.
There's much more to explain, but I don't feel like writing an essay.
I hope this solves your query.