Those of you who do not like Stargate: SG-1, skip this post because you will just be mightily confused. But if you like Stargate: Atlantis, any insight you can give me would be awesome ...? =D
So according to the Jaffa (and the Stargate movie, although at that point they'd only envisioned one other planet (Abydos)), Earth is the origin of all humans on all planets. The Goa'uld - or at least all the System Lords past and present - all have names that originate from Earth mythology. (You could argue that perhaps in some places, they created the mythology.)
I don't watch SG:A so I don't know if we have new information on the Ancients from that show, but I'm assuming we don't know where the Ancients originally came from. However, Earth does seem to have been their one-time Base of Operations within the Milky Way galaxy. From CotG and Solitudes, we know that all the DHDs (and how much do I love it that DHD is short for Dial-Home Device? Also that GDO stands for Garage Door Opener, hehe show!) share all their symbols except for the Point of Origin, which is probably unique to each Stargate (and may have once represented specific worlds, but the Stargates are moveable and there can be more than one on a planet). However, Daniel was able to decipher the symbols as constellations in the movie, so presumably these reference points are based on the stars as seen from Earth. Naturally, these constellations would not exist as we know them on other planets, so the Stargate system must be Earth-based.
(Like I said, I haven't watched any SG:A. Do the Stargates in that series have the same set of symbols?)
(And I've seen people argue that the constellations wouldn't make good reference points (Obviously this is true, since they're based on our two-dimensional perception of stars from Earth), but I'm going to handwave that with the sets of constellations being abstract identifications to differentiate between points in space - like how phone numbers are completely arbitrary sets of numbers assigned to a home.)
Now if I'm not mistaken, the presumption regarding Ra's rule on Earth was that when he departed through the Stargate for Abydos at one point, the humans in Egypt revolted, buried the Stargate, and lived happily ever after. Here's a good question for you: Why the hell didn't Ra come back in his ship and blow them to smithereens? Had Earth outlived its usefulness? I've seen it mentioned here and elsewhere that Goa'uld must have used the Antarctica Stargate after that, but then why is it still in Antarctica? Wouldn't they have moved it to someplace more useful than the middle of the coldest continent on the planet?
(Obviously Sokar made a trip to Earth in a ship sometime after the Giza Stargate was closed and yoinked a bunch of mideval folks to live on that one planet in Demons.)
Of course at the point that the Giza Stargate was closed to the Goa'uld, they had already transported humans all over the galaxy to work as slaves for them, and hung onto said planets with varying degrees of success; the Tollan escaped oppression and became one of the most advanced races in the galaxy, and other civilizations died out and others are still under iron fists. It appears that after the closing Earth became something nobody talked about, and probably was struck from every official record of Goa'uld exploits; while Ra was the one that was revolted against, it does appear that Ra's departure 5,000 years ago was basically the end of Goa'uld (ruling) presence on Earth.
Here's a thought: perhaps after the Giza Stargate was buried, when Ra dialed Earth he walked out into ... Antartica! Imagine his surprise! And then after that he probably declared Earth had returned to the Ice Age or whatever.
So 5,000 years later Jack and Daniel blow up Ra. There's any number of ways the Goa'uld could have found out about that, but Apophis moves fast. Let's make an assumption and suggest that Ra was the one that declared Earth off-limits to the Goa'uld after the revolt, for whatever reason. (Perhaps Sokar yoinked the mideval town in a fit of vengeance after he was deposed as a System Lord.) But now Ra is dead, all the power lines have shifted, and what better way to declare yourself the New Awesome Ruler of All than to go back to the forbidden Tau'ri, prove your superiority, and take back a prize? Apophis might be too casual in CotG for this to be viable, but he does seem to be hitting up all of Ra's old strongholds, so.
And thus Stargate SG-1 is born, hooray! We really should all be thanking Apophis.
And okay! Here's more thoughts! Do the Goa'uld at large have any reason to know how Ra was taken out? Apophis raiding Ra's strongholds suggests that they know Ra is dead, but not that they know how. Given the power struggle the Goa'uld go through, it would make sense that fellow Goa'uld would probably claim credit. Maybe Apophis claimed to be the one to wipe Ra out.
So then Amaunet rummages around in Sha're's brain and realizes that Ra was killed by people from the Origin Planet, Tau'ri. Apophis has seen with his own eyes how the Tau'ri people actually managed to kill some of his Jaffa when he came for the potential Amaunet host. Sha're may not understand the full implications of the nuclear bomb, but Apophis would, and he would put two and two together to realize the Tau'ri are actually kind of dangerous now. Add that to SG-1's exploits (and that early on they're really just discovering things and fighting Apophis exclusively for the most part), and Apophis has good reason to go wipe Earth out.
And that's all i have to say about that! Um.