My main computer currently is an Intel Core 2 E8400. I have been wanting to upgrade to a Nehalem core i7 860 or better, ever since it came out 1 year+ ago. Problem is every time I seriously think about upgrading to a new CPU/MB, I can't seem to justify the need for it. My E8400 seems idle most of time when I am using. Now I did use do some video transcoding for my handheld since can't handle 720P. But I quickly realized the best, quickest and fastest, solution to that would be buy handheld device that plays back 720P! Other than that I really can't thinking of anything I do that uses 100% of my CPU.
That said, I have a lot of other areas that could use upgrades. I keep on running of storage space, and what I really should do is build a 4 TB+ Raid10 for my videos. Also, potentially getting adding a SSD and more memory would be helpful. Getting a faster CPU is probably last thing that would have any affect on my computer usage. So I guess I should hang on to my E8400 for a long time?
If you have money you want to put into a rig buy an SSD. That is the first thing I would do.
Also, I agree with centralized storage. I like ZFS based systems myself, and there is a lot of good discussion in the Memory and Storage forum about that.
When I look at the idle CPU time on my i920 @4GHz, it's pretty much a waste unless you are a gamer who likes everything maxed out.