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Some motherboards require RAM to be installed in pairs, as you are doing now. Otherwise, there is no requirement to match the brand or speed of each pair, but the system _might_ run at the speed of the lowest pair. If they are all the same speed, they can be different brands if necessary.
You can easily tell the difference between 2 GB and 2.5 GB if you open many programs at once, it will be faster to switch between them as they can all be loaded into main memory at the same time.
Personally, I buy all my ram through cruical.com and I've never had a problem. Just remember to de-static yourself several times whilst playing with the internals of your computer, by touching some grounded metal (like a radiator).
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