APC is the recommended brand since they offer good products and good service.
Find out the power your system draws. You can estimate it by adding up the power of different components. Look in the user's manual of the CRT for its power. While doing that don't confuse power in Watts (W) with the VA rating. VA is not the same is W.
You also need to decide what you expect from the UPS. Do you intend to run a system for a long time when there is no line power (will need a very expensive unit) or do you need it to keep the system going just long enough so that you can save your files and power it down?
After you have these two pieces of information, you can go to the web site of the UPS manufacturer and look for run time charts like this one.
http://apc.com/products/runtime_for_extendedruntime.cfm?upsfamily=21
From this chart, you can see how long each unit will run for different power loads. Since you know your power and you know how long you want it to run, you can pick a device. keep in mind that you need to satisfy the first figure you found also (the maximum power that the UPS can provide).
The maximum power a UPS can provide has to do with the inverter that converts a DC voltage into an AC voltage and the output resistance and the efficiency of the system ....
The run time has to do with the capacity of the battery. You can add extra batteries to some units. That only increases the run time, not the maximum output power.
Edit:
I forgot to add that the UPS battery ages fast. In 2 to 3 years, you may need to replace the battery. That is another good reason not to get an expensive UPS. Then, you can just get a new UPS.
Automatic voltage regulation is a capability that some UPS devices offer, which you may want to have. But, again it will increase the price.
Cheap UPS devices are always off! They only engage and convert DC voltage to AC when the line voltage drops below the threshold. There are UPS units that are always on. This offers more control and consistency of the output voltage (for a price).