Hi guys
I read that the best version is the PC Engine Duo RX, that includes the cd rom, more ram, and can run the 7 games releases for the PC SuperGrafx.
Semantics mostly, but Duo R and Duo RX are EXACTLY the same internally; the only difference is the RX included a 6-button controller in the box. The 6-button controller works with any PC Engine, so the RX has no benefit over an R (though it's equally as good). Also, no Supergrafx games, but everybody beat me to the punch on that. Honestly, it's not worth the extra cash to put together a Supergrafx + PCE CD for the 5 extra games unless you're just a hardcore collector.
Also, two of the 7 games for Supergrafx (the two Darius games) will play on a regular PC Engine. The Supergrafx plays a slightly better version, but they do run on PCE as well. And yeah, the Genesis port of Ghouls 'n Ghosts is roughly on par with the Supergrafx version. If memory serves correct, the Genesis version fixes a glitch that doesn't allow diagonal input in the game, while the Supergrafx version does not fix said glitch.
There are also two different versions of the arcade card - there's the Arcade Card Duo, which is the cheaper option. The Arcade Card Pro, which is meant for regular PC Engine/Supergrafx consoles, seems to go for about 2x to 4x the amount of the Duo version. So, if you're interested in Arcade Card games, the Duo R or RX is the way to go, definitely.
There's also the Games Express card, which is needed if you want to check out the unlicensed CD hentai games. Not necessarily assuming you do, but hey, extra info never hurts.