The original A/B models have some problems with the USB ports and power.
For starters, they won't deliver much more than 150 mA each, and the USB 2.0 standard is 500 mA. This means certain things such as external drives will not work.
Generally, keyboards, mice, wifi adapters, and bluetooth adapters are okay. However, I believe many wifi adapters will be close to this boundary.
Further, inserting a device which requires close to the threshold (such as a wifi adapter) will cause a current surge and corresponding voltage drop, which browns the pi out: I.e., it will reboot instantly. That means these devices must be attached before you power up, and if removed, cannot be re-inserted.
To summarize, you almost certainly need a powered hub to try and do what you want.
attach a USB hub to another hub, without ruining the Pi
Considering one port is going to have this sub-standard limit, chaining hubs on it will be completely pointless. Any amount of things you want to attach requiring more than one hub will simply be too much -- unless, again, they have their own power supply.
is it safe to overclock an encased RPi to the maximum preset?
The major risk of overclocking is due to overheating, so you want to pay attention to your temperature (
vcgencmd measure_temp) when the pi is working hard, which will be often but not constantly in a desktop context. Make sure to install a CPU monitor to watch this so you know when to wait for something to finish, and if usage remains at 100% for a long time, check the core temperature. If it is exceeds 85 °C you are in trouble, and if it even gets close to that (say < 70) reduce your overclock settings. You are slowly ruining the processor.
The linux kernel on Raspbian defaults to a scaling governor, meaning the CPU frequency can be adjusted on the fly as needed. This means you can set something like:
config.txt and the governor will use 600 Mhz when idle and ramp up to 900 Mhz when busy. See the official docs about overclocking for more on this.
Normal operating temp for a pi is 35-45 °C, which may vary with a the ambient temperature. I've used scaling overclock settings like the above on a B, and when running at 900 Mhz, 100% CPU usage, even after 10-15 minutes in a ~25 °C room it does not reach 50 °C. There are monitors that will show both CPU usage and core temp (my preference is
gkrellm) which is useful for testing this.
Beware that part of the issue with overclocking is that chips that are supposed to be exactly the same may actually differ slightly due to factors in manufacturing, which is why some will perform better than others.