I think it depends on what you want from your consoles. If you just want to play your consoles on a TV that only has HDMI or doesn’t support 240p/288p, and you’re okay with having good (but arguably not the best) visual fidelity, then you could get away with using a RetroTINK 2X and cheap (compared to SCART) S-Video cables for all your consoles. That would also let you avoid modifying anything.
If you really want to use analogue RGB for everything, then what you do depends on the console:
- You already know the N64 needs to have an RGB mod fitted (because that console normally doesn’t do RGB output at all), and you’ll need to get an appropriate SCART cable for whichever mod you get installed.
- For the Dreamcast, you’ll want a SCART cable that has a built-in sync combiner and a 15kHz/31kHz mode switch, so you can get 480p out of it
- For the PS2, you just need an appropriate SCART cable. I would recommend cable without capacitors; I would avoid the cables with 1000uF caps that advertise compatibility with the PS1, PS2, and PS3, because those cables don’t meet the electrical specifications of either the PS1 or PS2/PS3. When using the PS2 with the OSSC, if you decide to play any games that use ED or HD modes (480p/576p and up), be sure to enable the OSSC’s automatic input switching; the PS2 switches from RGBS for SD content to RGsB for ED and HD content, and, without the automatic input switching, you’d have to manually switch between AV1-RGBS and AV1-RGsB using the remote.
- Since you have a PAL GameCube, you have RGB out of the box, but I believe you’re going to be limited to 480i for NTSC games and 576i (and PAL60 when available) for PAL games. If you’re fine with that, you just need a regular GameCube SCART cable, and I recommend setting the 480i/576i proc on the OSSC to passthrough to let your TV do the deinterlacing (assuming it supports 480i/576i over HDMI). If, instead, you want 480p for NTSC games, or 576p for PAL games, you’re either going to need an internal modification, like the GCDual, or an NTSC GameCube (Which, luckily, should be perfectly compatible with your PAL-region power supply and all of your peripherals).
On the high-end (in terms of quality and cost), there are HDMI mods for the N64, GameCube, and Dreamcast (there might be one in future for the PS2, but nothing so far), and, if you have a DOL-001 GameCube, there are HDMI modules that plug into the digital AV port (so they don’t require modification to the console to work). Those will provide the best visual fidelity, because they tap directly into the digital audio and video signals, rather than trying to redigitize the signals after they’ve gone through the analogue encoder.