Surprised no one has mentioned this yet. We use HelpSpot by UserScape (www.userscape.com) and it's been really, really great. Almost support ticket 2.0 (don't shoot me!). It costs money but it's pretty affordable -- the founder also frequents news.yc, I think.
Also, you will eventually outgrow using Gmail or Excel/Google Apps -- there's just a ton more features you're going to eventually need. Switch now, or switch later, eventually.
Things that hugely complex systems like RT completely miss: You really, really want to optimize on the bare-minimum path, IE what it takes to respond to a support request. Helpspot does this very well -- you can select pre-made answers that automatically file the request into a category and set any number of flags on the ticket (such as change who it's assigned to, etc) and that is really nice -- 1 click and you've answered a request.