Fighting Game and input buffering

How about you use a Queue<T>?

There are 4 significant members of Queue<T> you can use:

1. Queue.Enqueue(T item) - put your item at the end of the queue.
2. T Queue.Peek() - look at the first item without removing it.
3. T Queue.Dequeue() - get and remove the first item.
4. Queue.Count - check how many items are in queue.

Sample usage

Here's what could be a class representing moves:

public enum MoveType
{
    Fireball, Jump, Block
}

public class Move
{
    public MoveType MoveType { get; private set; }
    public double Duration { get; private set; }
    //...

    public Move(MoveType moveType)
    {
        this.MoveType = moveType;
        switch (moveType)
        {
            case MoveType.Fireball:
                this.Duration = 1.0;
                break;
            case MoveType.Block:
                this.Duration = 0.5;
                break;
            case MoveType.Jump:
                this.Duration = 0.8;
                break;
        }
    }
}

Your update checks to see if moves are elapsed:

Queue<Move> _moveQueue = new Queue<Move>();
double _elapsedMoveTime = 0;

public override void Update(GameTime gameTime)
{
    if (_moveQueue.Count > 0)
    {
        _elapsedMoveTime += gameTime.ElapsedGameTime.TotalSeconds;
        Move current = _moveQueue.Peek();
        if (_elapsedMoveTime > current.Duration)
        {
            _elapsedMoveTime -= current.Duration;
            _moveQueue.Dequeue();
        }
    }
    else
    {
        _elapsedMoveTime = 0;
    }
}

To add a move to the queue, simply call

_moveQueue.Enqueue(new Move(MoveType.Fireball));

To get the current move, simply

Move currentMove = _moveQueue.Peek();
if(currentMove.MoveType == MoveType.Fireball)
   //...

Also - don't count the duration in frames, count it in seconds (or milliseconds if you must).