Your 2008 model will not run Sierra, new hard drive or not. Suggest going to the App Store and upgrading to El Capitan OS X.11. Your current operating system is no longer supported by Apple with Security Updates which can present a real problem with 'net security, dealing with banks etc.
Both the 20" and 24" iMac can handle up to 6GB of memory, 1 x 4GB and 1 x 2GB despite what Apple said about the 4GB max. at the time of release. Well worth the upgrade.