There are a few sites that list free games for the Mac. These are some of those:
Jay is Games [http://www.thriftmac.com/Games/].
I recommend a nice free game called Battle for Wesnoth to begin your journey into the freeware for Mac. Very nice Turn-based Strategy game.
And now for the paying sites where you can buy Mac Games:
Apple [http://www.macgamestore.com/?gclid=CLyDuOi_4pwCFcIvpAodkQyGJQ] site itself.
Also Direct to Drive and Game Tap have a section of Mac games (not too ample though).
Recommended cheap games for the Mac (9 and 15 USD, respectively): The Path and Braid. Both indi games but widely praised. Just read a review to check if they are your thing.
You mentioned Spore. It is already out, and since the very first day it has been for the Mac and the PC (same box). I saw the box myself some months ago, so be asured of that.
RPGs is easy since they are a constant on the Mac. From Baldur's Gate (albeit this one is only for MacOS 9), through all Star Wars games, to Neverwinter Nights 2 (for present day machines), there have usually been Mac versions.
MMOs for the Mac are few, but they include WoW and WAR. The second one, Warhammer: Age of Reckoning, may be a condemned game and could close at any time in the near future (I hope it does not), but you never know. Just ask around before buying it.
To lift your spirits: Since the Mac turned to Intel chips, various developers announced, at that time, that making games for both platforms would now be very easy, and that they would certainly develop for both in the future.
And you will always have grandiose Blizzard. They have always developed for both, and most of the times they've come in the same box.
I hope that any of this information is of some value to you. CHeers. :-D