Arranging devices in rack cabinet

The cable managers are handy, but it's up to you if you want them or not.

The only real guides I have followed with setting up racks, is that it is best to keep the things you'll work on most and look at most around eye level.

If the server gets worked on regularly, consider getting one of those shelves for it and give it the best overall position. If not, put it down low with the patch panel.

You'll obviously be changing tapes in the backup, so make it easy to do so.

You'll wantto look at link lights and change connections on the switches, so arrange them conveniently. There's nothing worse than having to crouch down to try to see if you have a link light on amongst dozens of other link lights...

The router will most likely be telnetted to (at least thats what we generally do) and won't need much hands-on...so put it up top.

If you can leave a little space between components, this is good (for heat dissipation). Other than that...just make sure it's all secure.
If you add a UPS or something, that should go at the bottom...they are notoriously heavy. The worst rack accident I ever saw was a UPC being mounted up high, breaking free and crashing down onto the components below. *cringe*. That cost the company's previous consultant a job and was an ugly sight.

Good luck....

I agree with AlphaMale for the most part on having that which you use most at the most convenient level. We put the Servers on the bottom, router above that, tape above that, switches then patch panel, depending on how much space is left. The tape should be no lower than waist level with the switches and patch panel above it. You might go Switch/patch panel/ switch. Keep the weight down low and I agree that a bit of space between will help with cooling if you have room. Cable managers can be used as filler at the top, if you use it.

The thing about cabinets and arranging machines is that you should remember that most cabinets have a front AND a back!
You can mount stuff in the BACK of the cabinet, to increase space. Now if I did my math right, you have 34U used up (leaving 8Uleft) but you don't mention a monitor, keyboard or a mouse. 8U for a monitor should be ok, but I don't know about a monitor AND a 1U keyboard/mouse tray. That might be ok, but it is right on the edge of not fitting.
Anyway, I would arrange things like this, starting on top in the front of the cabinet, put the router/firewall first. At the very top. You aren't gonna mess physically with the router (only through remote sessions like Telnet or a software interface) so physical access isn't a biggie.
On the very bottom of the cabinet, in front, goes both Servers, taking up their 10U of space. Then, right on top of the servers goes the tape backup (2U). You are gonna change the tapes, so having them in front and a few feet off the ground is good.
This will leave the middle of the front of the cabinet empty. Monitor/keyboard can go here. Other routers/firewalls can go on the top, under your current router.
Now the back of the cabinet moun the patch panel first at the very top. Right below that goes 2 Cable Management segments.
Then right under that goes all of the STacked Switches, all 6U total of them. Place 1 Cable Managment secment between the switches, and the last Cable Management segment under the last switch.
I like patch panels in back, since the cable leads from servers and routers often times come out the back. So, it makes cabling cleaner with the patch panel in back.

Looks like you have several good answers already, so I will just chime in $0.015 worth.

Patch panels @ far top, since once they are connected, they won't have to be reached for often. In our building the cable bundles are all routed thru the ceiling anyway, of come to ground floor from upstairs...

Place switches adjacent to patch panels, for shorter cable runs, less excess to coil or hide in cable meanderer...

Power supplies & heavy battery backups (you didn't mention them, but will need some somewhere) at the bottom. I don't live in earthquake country now, but used to, so quickly developed the habit of not putting weighty, life-threatening objects on upper shelves...

Tape drives at/near eye-level for easy daily access w/o stooping.

Monitor, keyboard & KVM switch at eye-level (seated) for times you need to work directly on the server(s).

Cable managers - as needed. I believe the neater the environment, the more professional it looks. (not that I have always achieved that...). Even though the IT Dept is not directy involved in producing the products the company manufacturs, it can be a showplace where the VP of Sales lets visiting customers peer in, to "impress" them with "our" level of technology as compared w/ competitors. Plastic cable ties are cheap. Zip 'em, clip 'em. You can buy a lot of good first impression with $3 worth of plastic ties... I don't care much for the open hook-type cable managers you usually see. I prefer to use cable managers like my company uses inside electrical machinery we build, with clip-on cover to hide the cables, very neat & clean.

I took some pix of my server room "to show Dad where I work", and can shoot a couple over to you if you think they might interest you, or just for a rollicking good laugh at my feeble attempt to make it look like I know what I'm doing... :o)

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