would be going at essentially the same velocity as the payloads up until the moment of separation, which often involves springs. When you pop the payload off of the upper stage, there should be a slight slowing of the upper stage's velocity (action-reaction).

However, from that moment on, the upper stages are on a free trajectory with no guidance. They would've been heading in the general direction of Jupiter but without the ability to perform mid-course corrections, so it's unlikely they would've come close enough to Jupiter to benefit from a gravitational assist, much less come anywhere close to Saturn for a further gravitational assist. Short answer - the upper stages are probably either on a highly eliptical heliocentric orbit (perihelion near the Earth and apohelion out near Jupiter) or on a sun-escape trajectory but slower than the Vikings. My guess is that they're in the heliocentric orbit and won't be leaving the solar system.

Interesting question.