Exactly right.

If you have no problems with x38g, then probably stick with that.  x39 series was primarily adding diagnostic code, throttling back some risky areas for investigation on some problem cards/setups. 

x39e also winds back the default process priority to earlier standards, if you have problems with x38g normal priority that was meant to overcome Raistmer's OpenCL apps starving out the process in some cases.  Complaints meant going back to idle and stiff bickies if you want to run OpenCL apps in conjunction.

x40 is a now (recently) abortive, already defunct attempt at Cuda 4.0,  probably best avoided for most people.  Cuda 4.0 appears to have several performance & stability issues that likely only effect us, toward experimental optimisation etc.  nVidia's going to get a boatload of questions & reports from information gathered there.

I've reverted active alpha development to late x39 series, using alpha dev test pieces & information from Cuda 4.0 attempts to settle on final V7 code as Cuda 3.2 for x41 series, intended to largely consist of code aimed at the Stock Cuda v7 update.

Jason