I have two computers currently networked, however the route the cable has to take is approx 40'. This works OK, however one of the PC's (the elderly one) is being changed and the new one only comes with R45 socket. The new machine has R45 anyway.
I'm sure I have heard somewhere that there is a limit to the distance one can use R45/Cat5 cable without boosters. Does anyone know?
Cat5 cable can in theory be run for about 100m before you get any problems from it though generally the 100m limit refers to SOLID cable vs STRANDED cable. Stranded cable is generally used in patch cables, because it's more flexible and less prone to damage by bending, but it also cannot maintain signal over the same distance. Solid cable is usually used for premises wiring.
There's no good fixed distance I could find re: stranded; you can probably span 50-60 feet with no problem using it, but if you're approaching 100feet you might want to do something a bit more permanent with solid-copper cat5/5e/6.