Possible Scenarios for 64-bit: -- You have 32 bit OS, as such your amount of RAM doesn't matter. You're capped at ~3.5gb of RAM use and 64-bit won't gain you anything. You'll be unable to use the 64-bit KSP (it won't run) and you will have to keep using the 32-bit KSP program. -- You have 64 bit OS, but only have 4gb (or less) of RAM. You will be able to use ~.5gb more RAM on the computer than before (to your max installed amount of 4gb), and if your video card RAM was capped, it will have more wiggle room. Minimal improvement noticeable here if any, but you can still run 64-bit KSP. -- You have 64 bit OS, and have more than 4gb of RAM. You will now be able to use (unless you're a crazy enthusiast with 100s of GB of RAM) all of your system's RAM, and thus run more mods with (hopefully) more stability. Possibly a large stability improvement here if using a lot of mods and/or large texture files. Same video card improvements apply to GPU's with additional RAM to use. This is the people who 64-bit was made for. Don't know what you have? Right-click on "My Computer" and select properties.