You should change it to "Upgrade now!" I guarantee you this will perform better in A/B testing than "Upgrade !". As for a color, I'm not sure about that. I would simply make it a unique button on the top right portion of the page opposite "conquer club". However; if you decide to make that switch, change the layout to "Upgrade Today", so that it's less intrusive and a more natural fit to the site.
Also, it may be time to do some ppc. You have the market perfectly cornered anyway and whenever you get click through's it will cost you next to nothing regardless of the search engine you ultimately decide to optimize. You'll notice pogo is one of your only real direct competitors, and they're awful. Try google & yahoo! for starters.
By the way, why don't you have a "refer-a-friend" program? It would be simple to implement. Just offer 3 months free membership to referring friends and $5 off premium membership for incoming players. Unless this already exists and I'm just missing something.
Finally, you should consider updating your left panel side bar to the more "user-friendly" single buttons for facebook, twitter, digg, and myspace, etc. They are a bit larger, but no text, just the symbols. They also have a sleeker look. You're click-through rates should improve. Oh, and kudos on the twitter feed. Excellent stuff. Kudos!