Unfortunately, there is not a large selection for Mac at the moment.
There isn't? I currently own 356 games that all run in OS X 10.8.4 on my mid-2011 iMac. Nearly all of these games are reviewed as at least "good" on Gamespot or elsewhere. They include everything from wonderful old classics available on GOG to some of the latest greatest games from Blizzard, Feral, Aspyr, EA, Paradox and many of the very best indie developers.
Not to pick on you but geeze I get tired of people saying there isn't much selection on OS X. Yes, there most certainly is and there is good coverage of every genre one could want to play.
In response to the OP, if you haven't played the classic RTS Total Annihilation or even if you'd just like to enjoy it again in OS X on your Mac, you can find it for cheap on GOG.com along with a lot of other cool classic games for Mac.
I'd recommend installing Steam too for a large selection of games to play on your Mac and there is always the Mac App Store for more. Origin has a Mac client and features a good number of excellent titles from Feral there as well as some of their own titles, primarily Sims this and that.
I believe I have seen Supreme Commander 2 on Macgamestore for OS X.
You can also find Star Wars: Empire at War on the Mac App Store.
I noticed Black and White 2 mentioned above. Edwin from Feral has recently mentioned this will be coming to the Mac App Store sometime soon but as mentioned is available direct on disc if you didn't want to wait.
I also notice Tropico 3 mentioned above. It is worthy of note that Feral just announced Tropico 4 is coming in July and I'm sure it'll be on The Mac App Store as well. Most of their titles are.
I mention the App Store frequently here because I like it personally and it certainly is convenient being right on your Dock by default.
Edit: I forgot two more games of note for you. Company of Heroes: Campaign Edition from Aspyr is on the Mac App Store and they recently upgraded it so it now includes skirmish play. Wargame: European Escalation is another good game which is available on Steam for OS X.
But wait! There's more! Total War: Rome and Empire Total War are available on the Mac App Store and Napoleon Total War is coming out in July on the Mac App Store as well.
I don't think you'll have any trouble finding fun stuff to play on your new Mac.