Reason for build: Medium gaming/performance and a fair amount of multitasking - mostly RTS & RPG, not so many FPS
Q9400 (2.66Ghz stock) stable @ 3.4
Memory (DDR2-800 stock) stable @ 960
My PC built in 2008 (i think!) and upgraded several times over the years.
CPU was changed because my E8400 didn't overclock very well, could get it to about 3.7 but even at 3.6, 3.4 or 3.2 it wouldn't stay stable. The Q9400 overclocked much better and I got it second hand for $60.
Rebuilt the stock water block with a custom top during CPU change due to a possible fault with the standard one. Removing and re-fitting the original block can cause the clear (perspex?) top to crack/split over time from the change in tension - mine looked good but since I was changing the CPU anyway...
Hard drives were striped (Raid 0) and this setup offered more space (1TB) and was significantly faster than the original 150GB WD Raptor I had.
The case wasn't my first pick by any means, but I got it for a steal, they were selling for about $379 retail at the time and since I was working in the industry, a supplier's decimal error (ordering 10x too many cases) meant I got mine for $180. Winner winner chicken dinner.