outatime,
There are many ways to go with this.
Your low voltage lighting can be controlled by an Appliance Module or a Super Socket receptacle.
Your two 300W lamps are a little more complicated. I assume these are 300W halogens. Since you say you are "about to install" them, are they being controlled by wall switch(s)?
If you are controlling both lights with one wall switch, you might want to look a a Smarthome.com dimmable 1000W switch. X10 does not make a dimmable wall switch capable of handling greater than 500W.
X10 does make a non-dimming switch capable of 20A (WS13A). The WS13A switch must have a neutral wire present in the wall box ( a WS13A can also be used to control your low voltage lighting if you have the LV lighting controlled by a wall switch). The 500W dimmable wall switch (WS467 or WS12A) does not require a neutral.
In addition to the above you would want a controller to control the lights. You could use a plug-in controller such as the Mini controller, or the Mini Timer, or you could use a wireless controller such as a Palm Pad in conjunction with a tranceiver such as the TM751.