Me and my mates play both games at LANs and some of the former battles can last like a dozen hours at times. Even with newish Intel CPUs performance can crater late on in the games,and the RAM limitations don't help too.My favorite RTS games load the AI onto one core. Only way to get higher fps is to OC!
Sins of a Solar Empire: Rebellion
Supreme Commander: Forged Alliance
Both games are also 32 bit which means that if they hit 2gb of ram, they will crash. It can be fixed with Large Address Awareness to give it an extra gig.