I'm running on an inexpensive monster calculator:
Intel Q9550 Quad 2.8GHz +12MB of cache Overclcked at 3.4GHz with a Zalman Fan (45nm circuits are not very efficient with temperature, even at 3.4Ghz, the temp is not high than 45 C at 100% workload)
4 Gig DDR2 Memory (overclocked just a little bit to satisfy the CPU bus communication).
2 HD in RAID 0 for my Lightroom catalog and OS (with Raid on board controller)
4 HD in RAID 10 for my Pictures (and lightroom catalog backup).
I have been able to setup this pc for now much than 1200 $
OS : Windows Server 2008 64 bits (it was two years ago the best 64 bit OS to use with the PC).
I'm only doing Lightroom 64 bits on my PC, I have not other software installed instead of some Open source software : Gimp 64 bits for layer modification and Hugin for Stiching).
Less software on windows OS and better you have performance. (I'm not doing any game on my pc).
Probably Windows 7 will be the next step, but for now I no see reason to change from 2008 server. Windows 7 is base on the technology of Server 2008, they have rebuild the entire Kernel to satisfy the good management of processes (Inspired by a Unix or Linux, or let's say the good way of Mainframe computer of the old time). In the previous version of XP, and worst Vista, there was a very big problem to prioritizes the interrupt. On windows 7 or 2008, you can have access every time to the Process view and kill software stock on you PC draining the CPU. New kernel also manager better to share task to different CPU in multiple CPU architecture (DUAL or QUAD). XP was very stable at the SP2 and later release, but It's lack of good memory management and processes. The big problem of Microsuck
was to tried putting to much compatibilities on media interface on a short period of time, forgetting to invest on performance, the worst experience was Vista. We have done some test at my Job, and you can recycle old PC only by installing Windows 7 on it on how it very good manage now the processing.
On my actual setup, with 24MP RAW file from the A900, everything is fluid and I don't wait when I'm playing with slider adjustment. I hate when there is lag on a pc and I'm trying to use it. With good threading management, I can start Very huge batch of processing picture and continue to edit on other without feeling that there is plenty of calculation to apply my layer of changes on other pictures.
Next step will be to get a I7 Intel CPU , but for now they are a little bit to expensive. (but those guys rock with direct access to the DDR3 memory bus for calculation, what we needs when we are doing graphics manipulation).
There is also the possibility now to buy Acceleration Video card for game and send some Thread to the CPU unit of them to make new version of Adobe to use it instead of your current main board CPU. This can be a way to get speed instead of having to invest in a new CPU architecture (motherboard and CPU).