I have 2.4 Athlon AMD w/ 512 and an ATI 9700 Pro.
My 5 year old Mac a 350 G4 runs OS X just as fast as my PC runs XP.
To answer you questions:
There are G3's, G4's and now G5's.
G3's are a few years old and slow compared to the g4's and g5's, so don't even look at a used G3 on ebay.
There are many versions of the G4 that can be found at
www.apple.com or used.
The number before or after g3, g4, g5 is the speed like my mac (350 g4) that's a 350 mhz prosessor. You might think thats VERY slow compared to the speed of Intels and AMDs but it's on a different scale.
If your on a buget the lowest model G5 is SUPER fast compared to a Athlon 1.7. If you don't have the money for a G5. Look on ebay or on online Apple sellers for "G4 Tower" They are very nice also, you can get them in Dual prosessor configs and loaded with ram, DVD/CD burner, just like a PC. Also take alook at the imacs and emacs. Both great machines and faster then your current PC.
You could also put your 60 gig in your new mac for extra storage. Just an idea if you get a desktop.
If you want a laptop the ibook's and powerbooks are great and faster than your PC.
Your screen you use now will work fine with your a mac. So will any non PS2 mouse or keyboard. Macs don't have PS2 ports. You could always get an adapter though.
You will have to look at
www.apple.com for what computer and setup you would like and don't let the computer speeds fool you.
My PC and mac work great together, I use a router or hub. OS X (the newest and best apple operating system) has great networking options, share printer, share network (if you only have one IP), and file sharing between mac and pc works great too.
There is a long list of mac games. And normaly the mac version is released a few months after the PC version.
You can also get "Virtual PC for Mac, it allows you to run windows XP on your mac. You can run any program, kazza or whatever you want.
You will be amazed at how easy, fast, and stable OS X and Macs are to use!