If I empty the trash, will my iMac run any better?

I generally leave my Mac on 24/7. Should I be putting it in sleep mode or shutting it down? I was always under the impression that power on/off cycles do more damage than good. Can I set it up so it automatically goes to sleep at a specific time?

Yes you can set it up in the Energy Saver Pane of System Preferences - actually, it would be good if you took at look at all the Preference Panes there; that is where you set up how you want a particular app to respond. So, back to the sleep issue: in your manual, it says to let it sleep if not in use unless you won't be using it for a while (exactly what is 'a while'?). In any case, there are two sets of opinions: one to follow Apple's suggestion and put it to sleep (it is good to let your display sleep to avoid image persistance) and the other (which I belong to): if you don't need it, turn it off (preserve natural resources). My own personal opinion is that if I start it up/shut it down once a day, that is not putting that much stress on it - but I'm sure there'll be an opposing opinion. I've done it my way since I started with my first Mac in 1995 with no apparent damaging effects. But, again, there are two sides to this issue, so this will be your own decision. In any case, you can leave it on if you wish, let it sleep, or shut it down. Shut down does have some advantages sometimes: it performs certain checks and sequences which can sometimes fix some odd behavior.

It is good to repair permissions after any software update or install - the last update 10.6.7 messed up a bunch of them. You do this by going to Disk Utility (Applications > Utilities), highlighting your drive and hit repair permissions (no need to verify first). As long as it says "permissions repair complete" at the end, you're good (do not worry about some recurring messages there which have been showing up since 10.6.6). Also good to know: you can verify your hard disk there - if needed. If it needs repair, Disk Utility can handle a simple repair, but you'll need to boot from your install disk or a bootable clone to do this.

Just how many things do you have in trash?

There is absolutely no "make-believe" way to empty it. You hit empty - it'll be gone. Period. Now, if you choose to hit "secure empty", it will take longer because it will also write zeroes over the areas where the files had been so the contents can never be recovered - although it is just about impossible to recover a file even with the regular "empty" command.

Since hard drive space is not recovered until you actually empty the trash, you should empty it on a regular basis; I do mine at least every couple of days.

That alone will not make your Mac speed up unless it is taking up a very large chunk of hard drive space. Which brings me to the next question:

Highlight the Macintosh HD icon on your desktop and hit Command key and I on keyboard. In the resulting get info window, please post the following:

capacity

Available

Used

The Mac OS needs a minimum of 10 - 15% of total space to operate properly, so if yours is less, it's time to move some space hogging files (videos, pics, etc) off your hard drive.

Does the amount of apps on the menu bar or folders on the desktop have an effect of the performance?

When you say "menu bar" - you mean at the top of the screen or the dock? The menu bar at the top can be "personalized", but genrally, it's best to leave the default system icons there. The dock, however, has no limits - as long as space allows. I have 62 icons in the dock with my most used apps, files, and folders - it is primarily how I access anything. As for folders on the desktop - there are differing points of view: one is that every folder is treated like an app by the Finder when you boot up or restart, so it can take more time because it has to "open" all those items. Personally, I like my desktop as uncluttered as possible - I only "park" a few items there temporarily until I file, answer, work on, or whatever and then put them where they belong. I don't run my machine from the desktop - that is just the "gateway" to get you where you want to go.

You might also want to check in your Accounts Pane in System Preferences how many items you have set there to start at login - do you need them? That slows down the computer at startup as well. If not, delete them from that list.

Okay, so it seems you have plenty of hard drive space; however, it won't hurt to get rid of the 13+ GB in your trash. I would also keep track of your pics,music, videos, etc - you have quite a chunk there. For now, that is fine; however, when those files become even larger, I'd consider moving the iMovie, iTunes, and iPhoto libraries to an external hard drive permanently, freeing up space.

Do you generally shut down your Mac or let it sleep overnight?

Have you repaired Permissions lately (or ever)?

So just emptying the trash is not as comprehensive as the secure method of cleaning it out?

Yes, it is except for the fact that it does not write zeroes over the space - it just deletes whatever is there. That means if you don't use the computer at all after that and were to run a disk recovery app, you may or may not be able to recover some/all of the data. If you've used it (written over that space), not much chance you could recover much of anything. Whenever you do anything, it gets written to the hard drive; when you delete something, the OS finds that empty space to use for the next task, etc, etc.

Also, although it is generally assumed that browsers run fastest when they have old data in their caches, that can also slow them down. I regularly reset (Safari) and clear history/empty caches in Firefox. While checking those, you might also want to see if you want all those cookies every website forces onto your machine stored forever (I clear mine out regularly as well, only leaving my banks, etc. so they will recognize my Mac when I sign in).