1. Heat spreaders are not essential for memory modules.......they provide minimum performance gains (if at all) but due to the increased surface area, they do keep the modules cooler - which should translate to longer lifespan but not that we plan to use the same module for the next 10 years
2. In terms of latency, the lower the better..........its not that noticeable in real world computing (most of the time) unless you are making extra effort to detect the differences. Most of the difference will show in bandwidth benchmarks.
3a+b. Performance difference between DDR2~533 and DDR2~667 is not that big if at all. But since you already have DDR2~533 in your system, your new DDR2~667 might only run at DDR2~533 speeds. Unless your DDR2~533 is capable of running at 667 speeds, you can set it manually then.
4. PQI is already, they do offer excellent value stuff.....they do some white brand work for OCZ.....I guess thats some kind of an assurance in itself!!