It is not a cheapest, but...
nanoX-TCR Extreme Rugged™ COM Express ® Type 10 Mini-size Computer-on-Module with Intel ® Atom™ Processor E6xxT
http://www.adlinktech.com/PD/marketing/Datasheet/nanoX-TCR/nanoX-TCR_Datasheet_en_2.pdfCPU: Intel ® Atom™ E680T, 1.6 GHz, 3.9 W TDP
Memory: Soldered 1GB DDR2 at 800MHz (Optional for 512MB or 2GB)
Ethernet: Intel ® 82574IT Gigabit Ethernet Controller
Size: COM Express Mini,
84 mm x 55 mm (3.3” x 2.17”)
For comparison, most banking cards and ID cards:
85.60 × 53.98 mm https://en.wikipedia.org/wiki/ISO/IEC_7810But it is Atom. No HDD. No Case.
For example look at this:
http://www.adlinktech.com/PD/marketing/Datasheet/NuPRO-E340/NuPRO-E340_Datasheet_en_1.pdfNuPRO-E340 PICMG ® 1.3 Full-Size LGA1155 Intel ® Core™ i7/i5/i3 SHB
CPU: Intel ® Core™ i7-2600, 3.4 GHz, 8M Cache, 32nm, 95W TDP, LGA1155 (4C)
Memory: 2x 240-pin DIMM sockets / Dual-channel DDR3 1066/1333MHz, up to 8 GB
Operating Systems: Fedora™ 14, Red Hat Enterprise Linux 5
Form Factor: Full-Size PICMG ® 1.3 System Host Board
Dimensions: 338 mm x 126 mm (L x W)
http://www.adlinktech.com/products/index.php