System Specs

General

Despite the listing at the download page, the current specific requirements to run the game are not well known and are subject to change without warning. As of February 12, 2013, current demo and paid version are both based off of 0.18.2. Performance tuning can be expected as time passes.

Better hardware helps, but sometimes in non-obvious ways:

• A fast (per core) CPU. Dual-core may help, but given the limitations of the Unity engine, more will do very little.
• A dedicated graphics card (desktops & larger laptops)

KSP currently (18.2) does not make much use of multi core CPU's. You will get a better performance with a CPU with good single core performance and high clock speed. At some point the game also seems to hit a limit on whats possible so there's not much difference between 2 high-end CPUs running an extremely big space station.

The same is true of spacecraft design:

• Fewer parts, especially struts.
• Avoid highly detailed 3rd party parts.
• Keep installed part counts low.
• Use map view at low altitudes to avoid looking at terrain.

Paid and current Demo (0.18.2)

Minimum:

• Intel Core2Duo or equivalent.
• 2 GB of RAM (4 recommended.
• Intel HD3000 or better (Dedicated video card highly recommended).

Old Demo (0.13.3)

Fast Pentium 4 (>3 GHz) Fast shader model 2.0 card (3.0 or later recommended) that does not steal from system memory. Roughly an ATI Radeon 9xxx or nVidia FX/Gefore 5 series. 768 MiB of RAM

This is the absolute minimum to get a playable (if slow) game with smallish rockets, however. A faster system will greatly enhance one's experience.