Seeing as how some programs are already working on 4 cores it won't take much to use 8 - if you use those programs.
Too, its always nice to have extra power waiting for when you need it.
Yup, those would be the "highly specialized" programs I mentioned. Specifically, there's some image processing software that is very nicely parallelized, but most games aren't.
Second, buying capacity before you need it is a poor economic decision, because capacity gets cheaper all the time. Buy enough to last you for 12-24 months, tops; otherwise you're overpaying big-time.
Practical example: I built my current rig about 18 months ago. At the time, the top-of-the line CPU was the Athlon x64 X2 4800+. It cost a bit over 1,000 euros. So I bought the X2 3800+ instead, for about 200 euros.
I have not been CPU-constrained during these 18 months.
If it turned out that, say, Bioshock was CPU-bottlenecked on my rig, I could get an X2 6000+ for under 200 euros. I would have to change my motherboard as well, since the socket has changed. That would add another 100 euros or so to the bill.
So, to tally up:
Mobo: 100 €
3800+: 200 €
New mobo: 100 €
6000+: 175 €
Total: 575 €
Sell old mobo+CPU: -100 €
Final total: 475€
Had I bought the 4800+ 18 months ago, I would have paid about 1,200 € total, and I would have a slower CPU.
Lesson learned: whether you're buying disk capacity, CPU capacity, or GPU capacity, buy just enough to get rid of the bottleneck for the next 12-24 months, no more. Otherwise you'll just be paying through the nose for something that will be overtaken buy cheap low-end stuff about halfway through the projected lifetime of your box.
(Incidentally, my CPU runs current games just fine, so I don't even need to upgrade any time soon. Which means that I probably overinvested when I bought it... but only by 100 euros or so, tops.)
