A Berserker/Mage can be a great character. I'd dual him at 9th Level or 13th Level, depending on how many people you have in your party (which affects your rate of advancement) and how long you are willing to wait before your first class becomes active again. Here are a few suggestions for stats.
First of all, if you're playing a warrior, don't settle for less than 18/51 STR. The combat bonuses are just too good to pass up. This includes Fighters (single-class, dual-class, or multi-class), Rangers, Paladins, and Barbarians.
When you roll up a Fighter, you ought to be able to get 89 or 90 total stat points in less than 100 rolls. (I usually roll anywhere from 20 to 200 times.) When I first played BG1, I didn't realize that you could redistribute stat points since I had never done that before when I played D&D, so in my ignorance, it seemed really hard to roll up a good character at first; but since you CAN redistribute stat points, it shouldn't be too hard to create a decent character.
In order to dual to a Mage, your Intelligence needs to be at least 17. It doesn't really need to be any higher than that. As I explained in another message thread, the manual is wrong when it says that your Intelligence has to be 18 if you want to learn 9th Level spells. The game developers decided to change that at the last minute to give everyone a chance to use them. (I'm sure that this is true for Throne of Bhaal, but now that I think about it, I'm not so sure about Shadows of Amn. But no one is supposed to be able to cast 9th Level spells in the first place in Shadows of Amn, since Mages aren't supposed to be able to reach 18th Level.) If you're playing on "Easy" or "Normal" difficulty, there isn't any risk of failing to write a spell into your spellbook (on "Core Rules" and above, there is a chance of failure). There is a limit to the number of spells you can write in your spellbook, but you can use Potions of Genius to get around that (as well as prevent failure when you write spells into your spellbook). So if you have INT 17 and some Potions of Genius, you're all set to be the greatest Mage of all time.
I wouldn't settle for less than 16 Dexterity and 16 Constitution. If you plan to use ranged weapons, I'd bump up Dexterity as high as I could, since high Dexterity gives you ranged weapon bonuses. The Armor Class bonuses are nice, but if you're playing a character who can cast Stoneskins, Mislead, Improved Invisibility, etc., then I don't think that a couple of Armor Class points are really all that critical, and you could even get by with DEX 9. Constitution hit point bonuses are nice, but again, the same considerations apply. If you avoid taking damage, you don't need lots and lots of hit points in the first place. Ask any Mage.
Wisdom and Charisma are a good place to make tradeoffs. I don't think that Wisdom has any effect on anybody besides Clerics. The manual says that Wisdom affects your saving throws, but the manual is wrong, because that feature was never actually implemented in the game. You can raise your Charisma a lot of different ways, and it only has a limited effect in the first place. Even though the prices are higher in the stores if your Charisma is low, I never worry about gold because I do so much shoplifting and fencing.

You could also let a character like Keldorn (who has CHA 18) do your shopping for you. Charisma does have an effect on some of the dialogue options, but not nearly enough, in my opinion. So you can take advantage of a deficiency in the game and put your stat points somewhere else where they will make more difference. On a related note, there's one item in the game that raises your Dexterity to 18, so you might take that into consideration as well. Doing things like "min-max" and "meta-gaming" are considered to be cheesy, but if it doesn't make any difference to you, there's no reason why you can't use such strategies.
To preserve continuity (or to cheat, depending on how you look at it),

I usually assume that I used a couple of tome in BG1, since I always did. I didn't give every single tome to my main character in BG1; I gave them to various party members where they would be most effective. You might take that into consideration if you plan to edit your character. And speaking of editing, you can make your character as powerful as you like if you decide to use an editor; it's completely up to you (cheating or not).
And speaking of editing and rolling 200 times, you could save yourself the trouble and use an editor to give your character the stats you want.
To sum up, your character should aim for the following stats:
STR 18/51 or higher
DEX 16 or higher (9 if you plan to use the Gauntlets of Dexterity)
CON 16 or higher
INT 17
WIS 3 or higher
CHA 3 or higher
That's a total of 73 stat points, which you should be able to roll on the first few tries. If you get lucky and get 89 total stat points (for comparison, Imoen has about 87), you'd have an extra 16 points to distribute. That would let you raise your Wisdom and Charisma to 8 or 9 (the average for a Human) even if you raised all of your other stats to 18. You shouldn't have any complaints if you can roll stats like that.
