1. Motherboard - Due to the nature of the system being used as a file server, there are special requirements I am wanting for the motherboard.  Plenty of SATA III connections.  As the case has 4 3.5/2.5 internal bays and 2 5.25 external bays, I want the motherboard to have 6 SATA III connections.  With the SATA III connections, it also needs to support not only RAID 1, but RAID 5 as I do eventually want to have a large RAID array setup.  I also want it to at least use DDR3 memory and support at least 8GB.  If the motherboard does not have onboard video, it needs to have PCIe x16 for a modern card.  In fact, I want a full ATX board, that way all 7 of the expansion slot openings in the case have the potential to be used.
2. Processor - The basic requirement here is a 64-bit multi-core processor, most likely from AMD.
3. Optical Drive - Since this system will not be used to watch movies, but will need to load software from disc, a basic DVD drive will suffice.
4. Hard Drives - the dream list here would be a 250 GB for the OS and 4 4TB drives for a good high capacity RAID 5 array.  But for starters, the 250GB and 2 2TB drives would be enough to cover the work load my Dell has been performing.
5. CPU Cooler - NOT STOCK.  I'd prefer not using the cooler that comes with a processor, but a more substantial one, with a quiet fan.
6. Case Fans - The case already has one fan, but many places for additional ones.  These must be quiet and potentially long-life.  Since I do plan on using all 4 hard drive bays, a fan will need to be added to the currently empty fan mount in front of the bays.  Also required will be a fan in the back of the case.  Since the case fan mounts support 140mm, that's the size I'm thinking of.
7. Power Supply - This will definitely need to be a 80% certified, preferrably modular supply.  The rating will depend on the components used.  The Dell uses a 300W (or there abouts), but this new system will be drawing more power (eventually) as it will have more drives (eventually).  I'm thinking I may go with the 700W range, just to be certain the system has the power I need for future expansion
8. Video Card - This is one thing I will be cheap about.  As the system will be essentially a server, I don't need a card with tons of memory on it and killer graphics processor.  I just need something that throws out basic video to a monitor.  And this may not be needed if I find a motherboard with on-board video (something I usually avoid for gaming or even home theatre systems)
9. Memory - This I want to start with 8GB, at least DDR3.

Looking over this list, I see that I already have items 3 & 4 - DVD drive and the starter set of hard drives - straight from the Dell.  Also, if the motherboard does not have on-board video, the nVidia card in the Dell can also be re-purposed for the new system.

The motherboard is the key here that determines what processor, memory, and CPU cooler I will be using.  I already decided on full ATX, but what CPU socket?

I'll get into that next time.