You can use your existing Hi-Fi only if it has a line-level output. You cannot use the loudspeaker output (far, far, too high a level) and you cannot use the turntable direct (too low a level, and unequalised (wrong frequency/power curves)).
Other wise you will need a pre-amp or other device to connect the turntable to your sound card.
You can't do it with the two devices so far apart - the signal won't survive such a distance with any degree of fidelity. This applies whether or not you use a pre-amp or similar device.
There is a device to act as an input to computers for these applications - I think woodchip may know about it - I'm sure someone will.