Working on different projects I have the choice of selecting different programming languages, as long as the task is done.
I was wondering what the real difference is, in terms of performance, between writing a program in Python, versus doing it in C.
The tasks to be done are pretty varied, e.g. sorting textfiles, disk access, network access, textfile parsing.
Is there really a noticeable difference between sorting a textfile using the same algorithm in C versus Python, for example?
And in your experience, given the power of current CPU's (i7), is it really a noticeable difference (Consider that its a program that doesnt bring the system to its knees).
Is there really a noticeable difference between sorting a textfile using the same algorithm in C versus Python, for example?

Yes.
The noticeable differences are these

There's much less Python code.  
The Python code is much easier to read.
Python supports really nice unit testing, so the Python code tends to be higher quality.
You can write the Python code more quickly, since there are fewer quirky language features.  No preprocessor, for example, really saves a lot of hacking around.  Super-experience C programmers hardly notice it.  But all that #include sandwich stuff and making the .h files correct is remarkably time-consuming.
Python can be easier to package and deploy, since you don't need a big fancy make script to do a build.
Performance differences between Python and C