Two sticks will work, but with the fairly low price of RAM, why not add the extra stick?
because ram banks should be equipped symmetrically to take advantage of the dual channel architecture.
Also, RAM can slow down if the power supply is not quite sufficient for the system.
power undersupply won't make your system slower but unstable (random restarts and such things).
i would also go for a quad core cpu. i recently got a q6700 and it's great. not only for rendering and video encoding but also for games. for example physics (like newton) often can make use of multiple cores.
(i also only have a 450 watt power supply.)
