A bit of speculation and assumption, but I'm leading towards saying that they would have been able to build an Arena that had the majority of the functionality seen during the 74th and 75th Games.
There were still technological advances during the 75 years that the Hunger Games took place. Beetee (one of the Tributes in the 75th Games, and obviously a Games before that) was responsible for a number of them, such as the broadcast network:
"Our plan is to launch an Airtime Assault," says Plutarch. "To make a series of what we call propos--which is short for 'propaganda spots'--featuring you, and broadcast them to the entire population of Panem."
"How? The Capitol has sole control of the broadcasts," says Gale.
"But we have Beetee. About ten years ago, he essentially redesigned the underground network that transmits all the programming. He thinks there's a reasonable chance it can be done. Of course, we'll need something to air. So, Katniss, the studio awaits your pleasure."
Mockingjay - Chapter Three
That said, Panem, being comprised of the the Capitol and the Districts, existed before the Hunger Games came about. Unfortunately we're not given a timescale, but there's a brief bit of history at the beginning of the first book:
Just as the town clock strikes two, the mayor steps up to the podium and begins to read. It’s the same story every year. He tells of the history of Panem, the country that rose up out of the ashes of a place that was once called North America. He lists the disasters, the droughts, the storms, the fires, the encroaching seas that swallowed up so much of the land, the brutal war for what little sustenance remained. The result was Panem, a shining Capitol ringed by thirteen districts, which brought peace and prosperity to its citizens. Then came the Dark Days, the uprising of the districts against the Capitol. Twelve were defeated, the thirteenth obliterated.
The Hunger Games - Chapter One
So the timeline seems to be:
- Apocalypse (primarily natural disasters)
- Panem is formed
- Age of peace and prosperity (supposedly)
- The Dark Days begin
- The Twelve Districts are defeated, District 13 is destroyed
- The Hunger Games begin
Given the description of the "apocalypse" I think it's reasonable to assume that they weren't devoid of technology when Panem was formed. By the end of the age of peace and prosperity, at the very latest, they were developing nuclear weapons, so they must have been relatively technologically advanced at that point.
Haymitch won the second Quarter Quell (the 50th Games) using one of the force fields intended to pen in the Tributes, so that technology existed at least 24 years before Katniss' first Games.
I think the most telling thing is that the Districts rebelled. That suggests that the Capitol was already capable of controlling them prior to the Dark Days, their defeat, and the start of the Hunger Games. The Districts weren't a result of their rebellion, they were what they were rebelling over; they were just sent back there, with an even worse punishment added on top, when they lost.
Unfortunately there really isn't that much information on the past Games, especially the earlier ones. Those that are described, other than the two that Katniss participates in, are those that contained Tributes who are still alive, and even then there were very few details.