I think a left winger would be good for the squad, and also give a few extra options, but RvP can cover the position, especially in a 4-5-1, but also if Arsenal play the 4-3-3 Eduardo will play in that spot.
I don't really see that they need another striker, to be honest. It seems that because Eduardo isn't a direct replacement for Henry, and perhaps not the well-known name to palliate the idiotic potential Man Utd/Chelsea type fans, a lot of fans think that Arsenal will still buy another striker to replace Henry. It could happen, but I don't think Henry's spot in the squad needs to be 'replaced' anymore, as the dynamics of the squad have changed.
If they do buy another striker, it won't hurt the squad, but I think if they are going to buy anyone it should be a left-sided midfielder.