| Do you have broadband on the first PC? or do you envisage BB in the office?
If so I'd get a BB router a couple of Cat5 cables and n/w cards in the PC's if they haven't got them, and just share out folders via explorer.
If no broadband then buy a network hub, couple of Cat5 cables and n/w cards if needed.
Performance will be no different or not noticeable, and it will only be affected when X pc accesses Y pc and vice versa (obviously the specs of the PC's will affect performance anyway) | 
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