Hey, so I just picked this game up again because I really got in the mood...

And I started up in one of my games, where I have done most of the quests from chapter two (excluding anything with Umar Hills and Firkraag, so that includes the Sphere), and I was thinking about my party...

Right now, I have:
PC (Assassin)
Korgan
Jaheira
Viconia
Edwin
Kelsey

I like having Edwin and Kelsey because I love to have that much arcane power, and I need Korgan for a tank. Now, I need a healer, and I figure Viconia is better than Jaheira at that, but I like Jaheira better... and I need some ranged attack. Also, I want more xp, so I was thinking about dropping one or two characters.

What should I do? Should I have my assassin equip a bow except when I use her for backstabbing (as in, once I run out of invisibility potions and spells :p), or should I get Minsc/Valygar/Mazzy? Or should I not get any of them and also drop Viccy or Jaheira so I can get a bit more xp?

Any suggestions would be great, thanks.

How far are you in the Harpers quest? It yields good Xp especially when romancing Jaheira. Still according to your post I would drop Jaheira and take up Mazzy for her short bow skills.

You don't need range attackers - I mean they were good in BGI, but melee is so much more powerful than range in BGII.
PC (Assasain)
Korgan
Kelsey
Edwin
Viconea/Jaheira

I think Jaheira would be fine in terms of healing power because you only have 1 or 2 people who are going to get in close (and take damage). She also has access to druid spells but be aware that she won't have access to negative plane of protection and restoration (so save the scrolls). Not taking Jaheira or Viconia would also be doable given that most 3/4 of your party should never really be hit and there's plenty of healing potions/regen items to take care of Korgan. Note that this will leave you with only one tank, though this is definitely doable in unmodded BGII, and you'll always be at a higher level than you would with a 6 person party.

I have a few mods on, but nothing that adds too much to difficulty (I mean, a few new fights here and there, but nothing like IA).

I am not romancing Jaheira (my main character is female and I don't think I turned on the 'romance anyone' mod), so I won't get the 100k xp, but I like her because she isn't bad attacking (although her THAC0 is not great), and she has some interesting spells.

Thank you for your help :)

If it were me, I'd just stick with the party you've already formed. If you want someone to fill a ranged attack role, there are several in your party capable of that. How about Viconia with the best sling you can get her. And with two arcane casters, you're probably better off having the rest of your party doing primarily melee fighting. Perhaps Viconia using a sling when you really want a ranged attacker. Jaheira can also fill this role. Your assassin, or even Korgan can serve as decent archers in pinch.

I think your current party looks like it would be a lot of fun, and it's never a bad a idea to have an extra healer. ;)

Drop Jaheira, possibly Kelsey.

Edwin, Viconia, Korgan, and an assassin are more than capable by themselves. It's a fun party to play. You're playing evil -- ignore the lunatic, leave Mazzy in the jail, and kill Valygar (use his body to open the sphere).