Mostly the Greeks.
Romans were greatly influenced by the culture of Greece.
Palestine
There was no such thing as the Greco-Romans. Greco-Roman is a term which refers to art and culture. The Romans became strongly influenced by Greek art and culture and the term refers to the similarities which developed between the two, especially in art. This art had a strong influence of European art from the Renaissance to the mid-20th century.
The Romans have left us a large amount of beautiful art: statues, reliefs, mosaics, frescoes, pottery, silverware, jewellery, cameos and much more. Contrary to a widely held belief that the Romans were not good at art, they produced amazing works of art with a high degree of sophistication. Roman art was also multicultural. It is well known that Roman architecture and sculpture were influenced by the Greeks. It is less well known that later on it was also influenced art from other areas of the empire. The Romans liked to combine different artistic traditions.
the Romans added to their own talents and tastes to what they learned from cultures.
The Romans were a very innovative people, but one possible influence on their architecture would be the Etruscans, who are supposed to have invented the arch, beyond that most of what the Romans built was their own.
The art of the eastern and western parts of the Roman Empire were similar because in both parts of this empire it was Greco-Roman art. This is a term which indicates the fact that Roman art became heavily influenced by Greek art. Greek art had already influenced the eastern Mediterranean before it was taken over by the Romans because the Greeks had ruled this area for three hundred years prior to this. The Romans spread Greco-Roman art in the western part of the empire. It has to be noted that, however, many of the local peoples in the empire retained their traditional art.
I guess that by the river valley civilisations you mean Mesopotamian civilisations and the Egyptians. Greek pottery themes and styles were influenced by the Assyrians and Egyptians during the orientalising period of the 7th century BC. This term indicated that the Greeks were influenced by the art forms to her east, which also included influence from Anatolia (present day Turkey) and the Phoenicians (who lived in present day Lebanon) which were not river valley civilisations. The Romans were influenced by the art of the people they conquered, especially those who lived in the provinces. The Romans in Egypt were influenced by Egyptian art. They were not influenced by Mesopotamian civilisations because by then they had decayed and their area had been heavily influenced by the Greeks in the Hellenistic period (the spread of Greek influence in the Middle East and Egypt when were ruled by Greek kingdoms).
The Renaissance influenced architecture and art.
In art the Romans had excellent sculpture, mosaics, frescoes and jewellery. In literature they wrote good drama, comedies and satires. Some Romans writers, such as Virgil, Ovid, Horace, and Terence influenced European literature for centuries after the decay of Roman civilisation.
The Etruscans. They influenced Rome's daily life, government, and military, since they were the Romans.
Mostly from the Greeks, who influenced the Romans greatly in architecture and sculpture. The Etruscans were also an influence, but to a much lesser extent.