I too was recently confronted with finding a newer, more powerful system that would handle my photos while at the same time providing future upgradability. I looked at all the major companies (Dell, HP, Falcon NW, Alienware, etc.) and couldn't find a system configured the way I wanted for a reasonable price. So I ended up building a system myself. If you want a fast system, I recommend going with the new Core i7 processors (920, 940, or 965) with 8 to 12 GB of RAM. For an operating system, go with Vista 64-bit. Also, if you can afford it, you should get a fast 10,000 RPM drive for the OS and two high capacity drives for your data configured for RAID0.
Here are the specs of my new home-built system:
Core i7-965 processor (3.2GHz)
Gigabyte GA-EX58 motherboard
Windows Vista Ultimate 64-bit
12GB DDR3 RAM
300GB 10,000 RPM SATA drive for OS and programs
2 - 1.5 TB 7,200 RPM SATA drives in RAID0 for data
nVidia Quadro CX GPU w/1.5GB RAM
I've just run a test of this system against my old dual Xeon 2.8 GHz Dell Precision 530 workstation (Vista Ultimate 32-bit and 4GB RAM), and my new system blows the doors off the Dell. I work with large panoramas that range in file size from 500MB to 1.8GB. My test consisted of opening a 1.2GB file and running some typical actions on it (curves, flatten, resize, sharpen, and save using Photoshop CS4). The Dell ran the test in 4 minutes 55 seconds. My new system ran the test in 31 seconds. That's 9.5 times faster than the dual Xeon Dell!
So, my recommendation is to go with the fastest CPU and drives, and as much RAM as you can afford. More than 4GB of RAM will provide huge returns if you work with large files (>500MB). And if you go with a 64-bit CPU, then it makes no sense to go with a 32-bit OS, so I would recommend Vista 64-bit (Home or Ultimate - either one, doesn't really matter too much). Good luck!