If you want your consoles to look good on a big screen, you’ll need good cables to begin with. It will be tempting to sell your OSSC to buy those cables but as soon as you’ll get them, you will want to get an OSSC. My advice : don’t sell your OSSC, you will regret it.
And if like paulb_nl said, your Gamecube accepts RGB from the get go, then buy yourself a SCART cable so you get to enjoy at least one of your console with RGB and the OSSC. (I don’t play Nintendo consoles even though I’m French, I lost track of which console can output RGB without modification)
And if you can, plug the SCART directly in the TV, see how it looks and then use the OSSC and you’ll see that the OSSC makes a big difference and it should convince you to keep it.
RetroGamingCable_UK sells 3 cables for the NGC right now :
CSYNC : slightly better than the other one (mostly for PVM/BVM users) : £21.59 + shipping
Sync-on-luma (RGsB) : very good and the cheapest : £17.99 + shipping
PAKAPUNCH (CSYNC) : the best choice, fully shielded, important if you want to use a SCART switcher : £27.59 = shipping
Don’t be afraid to buy the cheap one, the difference between RGsB and CSYNC is honestly barely noticeable (especially since you don’t want the best of the best).
as a reminder
RGsB : R,G and B have their own wire, the sync signal is combined on a separate wire (sH+sV)
CSYNC : same as above but the sync signal is separated (H & V) and both have their own cable
Fully shielded : this one is CSYNC and each wire is individually shielded, so less video noise due to magnetic interference (useful when using SCART switcher(s))
My advice :
either the cheap one if you don’t plan on buying a SCART switcher. The PACKAPUNCH one if you do want to use a switcher.