Actually, the sim card holds cell tower routing info. So, while it doesn't directly affect signal strength, it is possible that an old sim may connect you to towers that are farther away. IIRC, there is some GSM command to reset the sim routing table, but when this happened to me, it was easier to just have the sim replaced. FWIW, frequent travel can cause this as well - there is a limit to the number of cell towers that each sim can cache.