I know the OP wanted to avoid buying separate amps for each room. I use a pair of compact Fostex powered studio monitors in the bedroom that sound great and plug directly into the Airport Express. This might be an option (although each speaker needs to be individually switched on).
One other consideration relates to interference in the 2.4Ghz wireless band. If you have things like cordless phones and microwave ovens operating, the sound can and will cut out occasionally. Therefore, you may want to look into using the 5.0Ghz A/N band for streaming audio to get the most reliability. However, the 5Ghz band doesn't offer the same range as the 2.4Ghz band, so you'll have to make a decision there.
Straight from Cisco:
"Digital subscriber line (DSL) is a modem technology that uses existing twisted-pair telephone lines to transfer high-speed data. Many types of DSL are used today; the most common are asymmetric DSL (ADSL) and symmetric DSL (SDSL). ADSL provides a higher downstream speed than upstream. SDSL provides the same speed for both upstream and downstream traffic."
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