Welcome! I personally stay Mac-only for gaming, though I do run Windows XP to test things for my clients. Some other people game mainly in Windows. You'll have to decide where you fit in that spectrum
A lot of big-name games ARE on Mac (I get game news from
http://InsideMacGames.com ) and run great. On my 24" white iMac I personally play Quake Wars, Quake 4, Doom 3, Prey, Unreal Tournament 2004 (soon UT3 and Gears of War) and Halo (out of print apparently), along with tons of great older games and great shareware. I haven't gotten much into war games, RTS, RPG, racing, puzzle, and many other genres, but those are out there too.
To answer you specifically: EA has started bringing games to Mac again--including sports--and WoW is available for Mac along with many first-person shooters.
But note that these games came first to Windows, and were optimized for Windows. OS X and Windows do things very differently, and optimizing for one isn't always ideal for the other. Mac porting houses do a good job, but tests often reveal Windows versions running a little faster. I don't much care: I can run these games with great quality and good speed, and have FUN, so numbers on a list of benchmarks are less important to me.
More important may be the fact that Mac games usually come out later, sometimes months later. If you need the latest thing NOW, get it for Windows or a console. And when the Mac version comes out, at full price, the PC version will already be on sale. I can accept that market reality--some can't. To me a game that was fun 2 months or 6 months ago is still fun.
Then there's the single biggest issue with Mac gaming: the title you want may not exist for Mac! Windows has a far bigger selection. I've been lucky: I like sci-fi games, and they have tended to make it to Mac. Macs don't have anywhere near the game selection of Windows, but they still have a lot of games (especially if you include all the great shareware).
If you're wondering about a specific title--whether it exists and how well it runs--Google for Mac reviews.
Be aware that some older games have been updated to Universal (meaning Intel-friendly) but the demos have NOT been. So don't judge UT2004, say, by its demo. Demo won't run well, but the real game does.
Boot Camp is the ONLY good solution right now for Windows gaming, unless you're talking simpler, less demanding games. Parallels and VMWare and WINE-based solutions have some limited ability to run Windows games without leaving Mac OS X, but it's not something to count on yet.
Re security: unless you stay off the Internet, running Windows can THEORETICALLY open your entire Mac hard disk up to attack. This has never happened to my knowledge, but there are at least two risks to even the Mac side:
1. Windows can't see the Mac partition, but a virus/worm/other malware can still see the physical drive device--and theoretically could erase the whole drive, Mac and Windows alike.
2. Software exists for Windows to let it read/write on Mac partitions. I virus could theoretically include such software, allowing you to mine or attack your Mac partition just the way Windows users have been attacked.
Again, that's theoretical--but still enough to give me pause. I'd rather not have to think about it. So I like VMWare. If you run Parallels or VMWare, games may not run, but at least your Mac stuff is safe: thanks to virtualization, your HD itself doesn't even EXIST to Windows in that case. (Unless you purposely share part of the Mac side with Windows. I don't: I just drag files back and forth.)
Now, aside from attacks coming from Windows and reaching the Mac side (highly unlikely) there is the issue of Windows attacks reaching the Windows side! Yes, you can get a virus in Parallels, VMWare OR Boot Camp, if you do something careless. There's nothing to stop that other than what every PC user does (or should do). Which means run a bunch of anti-malware apps that slow your system down and are a major pain when you start up. And which work great--until they don't. So you have to keep yourself educated--which is additional aggravation.
As a result, I avoid Windows as much as possible. OS X is stable, secure, and easy to troubleshoot. Games are already the LEAST stable class of commercial apps... the last thing I need is to be fighting Windows as well! That's not fun for me. And games are all about fun.
Another benefit of gaming in OS X, for me personally: it means I have my same browser, bookmarks, email, chat apps, text editors, and other tools at my fingertips. So when it's time to research a game question, gather other people for a game, download maps/mods/patches, or other activity AROUND gaming, I'm still comfortably at home in OS X.
Last but not least, I will game more if I can quickly and painlessly launch the game, play for a bit, and then get back to other things. Rebooting twice means delay and hassle, and I simply won't play as often if I have to face that. Even worse, rebooting means shutting down all the "stuff" I have open on my Mac, and then "getting it all out again" after the game. (Most Mac apps play well with others: you can leave tons of stuff open without a performance problem.)
And since I use my Mac as a PVR (via EyeTV) to automaticlaly record HD TV shows, rebooting would be a problem: the PVR software wouldn't be running, and if a show came on while I was gaming I would miss the recording.
Enough talk... back to Quake Wars in OS X
