So I received the potion of holy transference in malavons dome and I am planning on using it on my cleric/fighter who has 18 wis already but I was wondering what that would do other than improve his saving throws. In other words, would he gain additional bonus spells as he progressed after the wis improvement? If not I'll just sell it or reload to get the ring of reckless action.

Wisdom doesn't affect saving throws in Icewind Dale or the other games using AD&D 2nd edition rules. By raising wisdom from 18 to 20 you get more bonus spells, an additional 1st, 2nd and two additional 4th level spells.
But loosing one point of dexterity probably isn't worth it, especially if your AC and ranged thac0 worsens because of it (if your fighter/cleric has 9 dex it won't matter).

I consider the ring of reckless action the most powerful item in the game but you don't have to reload random drops in order to be able to beat the game.

Thanks, but I also have a follow up question then if you don't mind. I looked in the manual and am still confused as to what determines the progression of your saving throws. Is it constitution or do they just progress the same through levels depending on the characters class?

Little races get a bonus to 2 (gnome) or 3 (dwarf, halfling) of the 5 saving throws depending on con (+1 for 3.5 con) but the rest only depends on class and levels.