The PSP extended port, Phantom Kingdom Portable, has a fan-translation. Just figured I'd mention it. On a related note, I still wonder why, of NIS' PS2 offerings (the ones we saw the base version of out here anyhow), Disgaea, Disgaea 2, Makai Kingdom, La Pucelle, and Phantom Brave all got extended PSP versions, but Soul Nomad has remained PS2 only (and only saw a PSN digital rerelease in Japan).
Does .hack qualify? Then that.
I'll give it that it's not all that popular (especially compared with the likes of Final Fantasy, Kingdom Hearts, Dragon Quest, and Tales these days), but I wouldn't exactly call it a hidden gem, or all that unpopular either. More likely that there's a decent amount of people that know of them, but haven't bothered to get invested in the games due to the pricetags on them if one doesn't know how/isn't apt to pirate games. From what I've seen locally, Infection being easy and cheap to find hints at it having done rather well in sales, but subsequent releases either saw less shipped or less sold in general (the multi-installment "pay three/four times for one full game" release style understandably being a turn off). Though, with Namco having given CC2 clearance to make a PS4 remaster of G.U. with various gameplay tweaks, and apparently an entire extra volume of new content, the series may yet see another day in the limelight (especially with the PS4 version being an "all-in-one" rerelease, and for $50 at that). Just my thoughts though.
Who I feel sorry for now is the people looking to fill out their GCN collection these days; prices for those games are insane now. GCN game prices at my local indie game shop start at like $70 and go up past $100, for anything that isn't literal shovelware.
It's certainly been a slow effort on my end to just acquire the thirty or so Gamecube games I've managed to get without paying the average prices, which can indeed be downright ridiculous. However, while it's certainly still a "scalper" system out here, the prices where I am thankfully do NOT start at $70 for anything worth playing, and there's a handful of very worthwhile games that can be acquired for $10-20 each. Still a problem though that ultra common (for the system) games like Mario Sunshine, Windwaker, Pokemon Colosseum and Gale of Darkness, and Melee can be $35 or more (Melee being $55+ here) because they feature major Nintendo characters, and of course the actually hard to find games are going to get scalped these days, especially if they've managed "cult classic" status since then. Even multiplats can be a good twice as much for the Gamecube version as the PS2 or Xbox ones.
PS2 has definitely been easier to pick stuff up for in my experiences, and has been more prone to stores having pricing fuckups I can make use of than what they carry for the Gamecube. Also helps that various companies have been willing to do reprints over the years, helping reduce aftermarket prices and increase availability on previously rare, expensive games (though admittedly there's still a fair amount that have never seen that treatment and probably never will).
One of the things to bear in mind is that you can't just place ISOs on the hard drive and have the system be able to use them. You have to go through third party applications (WinHIIP, HDL Dump/Dumb) to copy them onto the drive and have them be readable. This isn't too bad, but it can take a bit of time; connection failure during large installs can be damn obnoxious, hence my warning there.
Also, I'm not sure if you're planning to use a modpack when setting up and formatting the drive (it's a good idea for the base utility; having uLE preinstalled is helpful), but make sure you update any loader applications, as various game that wouldn't play well/play at all before may work much better with a later version, and from what I've heard there's some compatibility variance between HDL and OPL (I mainly use the latter).