Yes, I am worried about that too (adding an extra computer, since in my experience they are some of the worst noise-offenders, along with television-sets. At least those 2 device-types are those that make most of the noise I find here in my setup)
I am currently testing a 45' (15 m) antenna-cable, and the results are good.
I accidentally discovered how noisy a USB-extension cable really is (or can be, at least), since my initial routing had the antenna-cable laid out along a USB-extension (an extension used for other purposes)
Whenever the computer was using the USB-device, it would create huge spikes all over the spectrum.
I solved this by making sure the 2 cables are not near each other.
But it was a clear example of just how much noise USB creates (or broadcasts)
The main potential issue I feared I would have with the long antenna-cable was a loss of signal (lower SNR), but I don't observe that with the current length.
I did actually try an amplifier at the antenna side, but it did not improve the SNR (in fact it reduced it) even though the overall signal strength was boosted.
So it seems that this antenna-cable length is not long enough to cause any real problems.
How much noise it picks up is of course difficult to measure, since I can't make a comparison-test without physically moving the computer closer to the antenna (which is more involved than I want at this point)
But I assume (always dangerous to do though

) that it would likely be worse to have a long USB-cable than a long antenna-cable, since a long USB-cable would, by default, take the computer-noise closer to the antenna (as per the accidental discovery mentioned above)
At the moment only the antenna-cable is 'linking' the computer and the SDR, which means computer and USB-cables are not in the same room as the antenna.
Had I used a USB-cable extension all the way I would obviously have the SDR and all the USB-noise in the same room as the antenna, and my assumption is that that would probably have made things worse (now that I have seen how much noise USB really broadcasts)
So I dare conclude: longer antenna-cable is better than longer USB-cable
Thank you all for your input

jacob.