I'm a long time mac user and am currently using a late 2008 model Macbook with Snow Leopard. Android phones do not require much interaction with your computer.
Contacts
All of your contacts go through GMail, so I would recommend setting up that account and getting all of your contacts sync'd before you get your Evo.
Video Out
Your iMac if I'm not mistaken only has a displayport output, no type of input, so you won't be able to plug your Evo into your iMac monitor without some 3rd party type solution (I assume one exists, but I have no idea). Obviously if you have an HD tv you can plug into that.
Apps
All of your apps don't sync through anything like iTunes, they are all done over the cloud, so no worries about Mac compatibility there.
Music
If you are planning on putting music on your Evo I would suggest downloading doubletwist. It can sync your itunes library/playlists to your Evo without any problems. Or, you can just go for the drag and drop method. Plug your Evo into your Mac, select Mount as Disk Drive and your phone's sd card will appear on your mac just like any usb drive would. Simply drag and drop files to it.
Ringtones
Here's the easiest way to set up your ringtones. On your phones sd card, create a folder called "media", inside of media, create a folder called "audio", inside of audio, create three folders "ringtones", "alarms", and "notifications". Then just put your corresponding files into these folders. Now when you go into your Evo settings and select a ringtone/alarm/notification your sounds will be there. There are other ways of doing this, namely using an app like Rings Extended which will let you pick any audio off of your sd card for ringtones/notifications/alarms, but I prefer the neatness of the method I just described.
So in short, your experience on a mac will be no problem.