Bueno no uso Paradox, solo MS SQL Server, haber si te sirve este codigo:
Código SQL
[-]SELECT camp1, [campo nuevo]=
CASE camp2
WHEN 1 THEN (SELECT SUM(t.camp3) FROM tablax t WHERE t.camp1=tablax.camp1 GROUP BY t.camp1)
ELSE (SELECT SUM(t.camp4) FROM tablax t WHERE t.camp1=tablax.camp1 GROUP BY t.camp1)
END
FROM tablax
Saludos