It's a while since I bought my desktop PC. I tend to keep kit a lot longer than most of the people I know, indeed I still use kit that is older and slower than what some people throw away!
My desktop system is now getting too slow and full. I would never say I'm a good photographer but I do take pictures and the ancient 120 GB drive on my desktop system is full. I've got a lot of that backed up on my home server but even so the drive is just full. It also doesn't have enough memory or CPU grunt to edit a decent sized RAW image in GIMP. It's swapping far too much.
The best thing to do is buy a new box. I think that a 180 GB flash drive for root and a 2 TB hard disk for bulk storage should hold the OS and my pictures for a while. RAM is reasonably cheap so I'll probably go for 16 GiB off the stuff which should allow the system to breath more than it is with just 2 GiB that it has now. Not sure on the CPU, the quad-core AMDs are pretty cheap and come with a decent built in GPU but the twin-core Intels are faster if more expensive.