netvampire:
In general you'll probably need the following components. Prices are rough estimates of "street" prices.
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Computer interface: ActiveHome Pro CM15A ($50) or possibly the old ActiveHome CM11A ($10-$30). You may be happier with the newer and fancier CM15A.
Plug-in Lamp Modules, e.g. X-10 LM465 ($10)
Wall switch dimmer modules: X-10 WS467 ($10), or more mechanically-rugged models (about $30) (Go for the more rugged models for switches that will be frequently operated manually - the WS467 are reported to have problems with the push-button wearing out. Or just take your chances with the WS467 and replace when/if you have a problem). There is also the WS4777 if you want to replace any 3-way wall switches in your house.
Passive phase coupler: ($20 for a model from Smarthome that will plug into an electric clothes dryer socket.) However you may find you need a coupler/repeater ($80) if you have a big house.
Transceiver - for whole house operation you'll probably need an X-10 TM751 ($12) or X-10 RR501 ($20), or possibly even a WGL V572A All-Housecode Transceiver ($80) if you have a very big house. Although the ActiveHome Pro CM15A has an all-housecode transceiver function, it's not noted for its RF receiving range. The TM751 and RR501 are limited to a single housecode (16 units).
Remote controls, e.g., X-10 HR12A ($4-$20) (One or more depending on your preference.)
Optional plug-in table-top controllers, e.g., X-10 MiniController ($12) or MaxiController ($20) (Good to have at least one of these on hand.)
Possible X10 Noise Filters ($10-$15) for appliances with switching power supplies like TV, UPS which may generate noise or act as an X10 signal sink on the power line.
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X-10 runs frequent sales with very good prices on bundles and kits, and 3-for-1 to 6-for-1 sales. Otherwise check the X10 stores operated by small authorized X-10 dealers on eBay who are selling new parts.