I'll be building a computer for a friend in a couple of weeks, whose most strenuous task will be to run a statistical package she needs for her Ph.D. research. The program actually recommends 8 gig of RAM. Does it make much difference if she buys 2 sticks or 4? I highly doubt she'll ever need 16 (and if she does, would probably have a grant that could pony up for it), so I don't think it matters if there are no RAM slots free on the mobo in the near future.
Generally the default BIOS settings are for two sticks of RAM operating at the board's stated speeds. You can change the settings, and often with four sticks in a board with 4 DIMMs, you will have to increase the northbridge voltage a bump or two from the default settings to get the system to boot.
Also, your RAM may require you to change the default BIOS settings to achieve the advertised speed and latencies. Some BIOSes have a feature to enable the RAM's XMP profile, so you do not have to change the specific RAM voltage, frequency, or latency settings.