Reliability of cheaper RAM like Komputerbay, or unbranded?
I have used RAM from "tier 1" vendors, as well as from "value" vendors like the ones that you've mentioned. I've had very few sticks "go bad over time" or "just die", but I tend to buy DIMMs (dual in-line memory module) appropriate for the application. 

This last bit is key: RAM from a vendor who manufactures a quality product (good solder joints, board integrity, etc.) typically has problems when used outside of its intended parameters, generally defined by inconsistencies in speed, timing, and latency (the gap of time between when the computer attempts to read/write/act on the RAM and the resulting change).

You can install slower RAM in a faster computer. In fact, many computers tend to run slower, adjusting themselves to the RAM's timing. If you overclock any part of the system (processor, RAM, video card, etc.), the component needs to handle the additional thermal demand placed on it. RAM that cannot dissipate heat effectively enough will eventually have future problems, as the continual exposure to too much heat causes some parts to melt or degrade.

Just because "Vendor X" has RAM priced at an absurdly low (or high!) price doesn't mean that it's a good deal. Just make sure the RAM meets your system's requirements. As @lupincho's comment states, using the vendor's "configurator" will get you the appropriate components for your system, and a much greater chance the vendor will stand behind their warranty.