Couple of things. RAM reduces when you are using multiple applications, having just SWBF2 open should not consume that much RAM.
Systems with faster RAM, good graphics cards and a decent motherboard should be more than able to handle SWBF2 given it is an aging game.
Lag is a key issue, ISP, Firewalls, AV software and any background processes from other applications can factor in. Be careful in ending processes as they could cause your system to become unstable. Use an application like process explorer to take a look at what is running before you leap into anything and do any in haste.
Regards,
Kren