I think for a good attack there needs to be a balance of flair and discipline, with work rate thrown in to play a high pressure game in the opposition half. Our current front four have discipline, each of them knows their jobs, but is perhaps lacking the flair required to break down tight defences, so we look to players like Silva to strengthen our attack.
The Barcelona strikeforce excels in terms of flair and creativity with players like Iniesta and Messi, but they also have a workhorse in Eto'o and a player with huge experience in Henry to remain disciplined, which is why they scored in excess of 100 goals last season.
Looking at the new Real Madrid frontline, there appears to be a lot of flair, in Ronaldo and Kaka and maybe Ribery, but I'm not certain if they would be prepared to hold their positions for the sake of their team mates. Ronaldo will certainly see less of the ball than at Utd so maybe he will get frustrated and roam around and try to be the hero with no-one to cover his position without the presence of a Kuyt or Eto'o, leading to a disjointed attack, easy pickings for defenders. With none of them prepared to do legwork, you will lose some of those defender-drawing runs that people like Kuyt makes, and defences will be able to shut them out. I can also see ego getting in the way of team play, with everyone trying to win each game on their own - would Kaka always give the ball to Ronaldo like Carrick and Anderson? Whether their frontline works remains to be seen but big names do not equal success.