Hi

I've recently started playing Icewind Dale II with a party of 6 characters and is in the first chapter, outside the goblin fortress.

The spellcaster in my party (beside the cleric) is a sorcerer, because I've always like the class - partly because of some pen 'n' paper sessions I've played in with a great sorcerer, partly because I like having plenty of spells without having to choose.

However, I am considering dual classing him to a wizard to get a better spell selection (having the right spell just for a tight spot, etc.) and to be able to learn spells from scrolls.
Is that a good idea? How will it work with selecting spells? will I still have the large amount of spells from the sorcerer class available?

It's not generally a good idea. Every level you take in Wizard delays your development as a Sorcerer, and vice versa. This means you'll have access to many lower level spells, but you'll be missing out on higher level spell casting. For example, a SOR 6/WIZ 4 can cast up to level 3 SOR spells and level 2 WIZ spells, while a SOR 10 will have access to three level 5 spells.

As well, Wizards use INT for spellcasting, not CHA, meaning you'll need high scores in both in order to make your spells hard to save against. Finally, (IIRC) you still won't be able to scribe spells for your Sorcerer side to cast. (i.e. if you learn Fireball from a scroll, you won't be able to slot it into your Sorcerer spells, only your Wizard spells - I'm not sure on this. I know it works this way in NWN, but can't recall if it's the same in IWD2. I assume so). You're far better off remaining pure Sorcerer, concentrating solely on CHA, and blasting away, IMO.

Dual classing from sorcerer is a very bad idea. Multiclassing your spellcasters is a sure way to cripple them.

Do a comparison at level 12 for example:
While a level 12 sorcerer can cast spells up to level 6 with casting level 12 a sorcerer6/mage6 is limited to level 1-3 spells with casting level 6. The game will be hard to beat with low level spells when you're supposed to be able to cast powerful high level spells.

If you select your spells at level up wisely your sorcerer won't have trouble covering the arcane spellcasting. If you want to try out other spells there's still the option to use scrolls directly.

If you want to have more arcane spellcasting consider giving caster levels to one of your non-casters with some int or cha, rogue/transmuter is a very popular mix among those who don't use sneak attacks.

The message seems to be pretty clear - I'll refrain from dual classing my sorcerer.

I might dual class my rogue, but that's a different story entirely.

Thanks for the answers, both of you :]

Rogues are good to multiclass if you multiclass them to ranger with 1 level for twoweapon/ambidex feats. After that, they are better with more levels in rogue for extra sneak attack damage. If you make a rogue focus on multiclassing into a fighter then you lose out on a lot of sneak attack damage but it might be worth it for the extra health, damage and bab. Aside from that, rogues are good just as they are.