a US System cant play Japanese games and vice versa.
In terms of Duo systems, (keep in mind ANY region's CD system can play ANY regions CD games...no limits!)
1) the Duo-R or RX is best to locate. You can play Japanese hucards, and any CD game.
plus, you can get the arcade card and play Arcade CD games.
then, you can get it region modified to play US Hucards also.
2) buy a US Duo (which has sound issues that need fixed due to the age of the system and stuff) and have it region modded to play japanese hucards, and then you can do the same as 1)
3) Get a US Turbo-Grafx 16 + CD add on, and again, get it region modded to play Japanese huCards (or use a Hucard converter) then buy a system card 3.0 putting you back at doing what 1) can do. (Or get a Japanese PC Engine + CD add on and region mod to play US cards...)
these are the best options.... ranked from BEST to WORST
Once you region mod a US based TurboGrafx system, you can get a cheaper Japanese sys card 3, since the US version is expensive/rare.
Another option (what I do...) I have a Duo-R to play Japanese games + CD stuff, and then a US TG16+CD to play US games. takes up more space, but skips having to region mod your system.
Gamewise my favorites:
Shooters: Fantasy Zone, Lords of Thunder, Final Soldier, Psychosis, Tatsujin
Puzzle: puyo puyo CD, Skweek, Puznic, Panic Bomber
Platformer: Parasol Stars, any Bonk game, Shockman, Kaze Kiri, Ninja Spirit, Dracula X
Check out pcengine.co.uk for more stuff!