Someone told me that recording a sound really makes a huge discrepancy with a good OS , soundcard???
The sad truth with most consumer operating systems today is that they are not optimized for the true 'real-time' requirements of multimedia recording, be it windows or macos or linux, although the latter can be optimized if you strip it down to just the needed components. There used to be tweaked versions of XP where you removed all the non-essentials such as networking, internet, and certain hardware not required for multimedia. For example, why put in a powerful video card when you are specializing the computer for audio? Video card drivers have been known to use up every last bit of cpu headroom just to achieve high frame rates.
Next, having great drivers are very important. Standard windows drivers are so-so; they were just meant for good audio playback with low regard for latency. Asio drivers appear to be the best standard nowadays so any audio card that doesn't support asio is almost always a 2nd or 3rd rate device.
If you will be using your pc for more than just audio, be aware that there will be compromises, but at least try to get the best audio interface you can afford that supports true asio, not asio4all. Next, get the best cpu you can afford and pair it with a very good motherboard. Another important item that is often overlooked is the power supply - get one that isn't generic and is more than capable of powering up your system. You will need the added power when you have several drives. Like I always save, have at least 2 drives, one for the system, and the other for your audio files.
When it comes to ram, 2GB used to be the max, but with win 7, it seems 4gb is now the minimum. More if your are using 64-bit but not much higher if you don't use ram-intensive programs such as samplers. Get good ram, not that generic stuff and make sure you get ones with good ratings. Yes, those X-X-X-XX X.XXv CL ratings mean something and pay attention to the speed of the ram. You don't need the highest speeds and lowest ratings, those are freakishly expensive, but don't get the cheapest ram either.
Finally, get a good case with good, silent fans and excellent ventilation. You don't need case and cpu fan that sound like jet engines in full afterburner everytime you fire up your workstation.