We've built 4 new computers in the office (to replace core 2 duo windows xp based machines) and having spent some time researching the different options we decided to pair a low end AMD CPU (based on the "Trinity" architecture) with an Asrock motherboard and Intel SSD.
We wanted a low cost system that could encrypt files reasonably fast, with a future proof upgrade path and low power consumption and we think the build detailed below hits the mark for our specific workloads.
AMD A4-5300 CPU; €37.00
Asrock FM2A75M-DGS Motherboard; €53.00
Fractal Design Core 1000 case; €28.80
Intel 330 120GB SSD; €71.50
1 x 4GB Dimm ddr3 memory -1600MHz; €15.00
Antec Earthwatts 380W Power Supply / Be Quiet! 300W psu; €35.00
Total Hardware cost: €240.30 *Notes: No CD ROM, Build time approx 20 minutes, Price net of 23% Vat and delivery charges
Intel also have excellent budget options (with higher single threaded performance) and I would recommend the Anandtech value system buyers guide by Zach as a good place to start.