You can store them in a list for example:
List<string> valueStrings = new List<string>();
string value, valueString;
while (x.Read())
{
value = (string)(x["ROWVALUE"]);
if (valueString.Length + value.Length > 300)
{
valueStrings.Add(valueString);
valueString = value;
}
else
{
valueString += value;
}
}
valueStrings.Add(valueString); //don't forget about the last one