The LGA775 3000 Series Xeons are dedicated server parts designed for single-CPU entry-level servers. The 3200 series chipset they use only supports up to 8GB of DDR2 and systemboards based on them are $300-500.
They are cheaper then the 5000 series used in the Mac Pro, but they have higher TDPs so they'd be poor choices for a small tower. Also, the cheapest models only have 4MB of L2 cache which will hurt performance. So you'd still be looking at upwards of $1000 just for the top-end 2.83GHz X3360 and a systemboard.
Assuming Apple wants to move to desktop parts (and that is a big assumption, to be honest), Apple could do a mini-tower about the size of a Shuttle Computer cube (the
D10 would be an interesting idea). Of course, something like the
SX48P2E is $450, and then you need to add CPU, memory, HDD, CD-ROM and video card.
So say a "Cube Deux" with a Q9450, 2x1GB of DDR3, 8800GT, 500GB HDD and a DVD-Writer would run $1499 (based on the parts price and a 30% margin).
If Apple wanted a smaller case, using mobile parts, figure $1799.