Lak's got the right idea. Using a larger transformer to filter the entire system has many advantages and it is what i would recommend. This keeps the system at the same voltage potential as all of wiring passes through the same transformer.
On top of that, this moves the transformer away from the listening area. Any / all transformers generate some form of noise & EM field, so the further it is away from your listening position and equipment, the better off that you are. Placing it in the room where your main AC system feeds your sub-panel is the best way to go, both sonically and in terms of ease of wiring. Listening to a very low level transformer hum in your room does nothing to help the noise floor that you just invested tall cash trying to achieve.
As far as selecting a transformer, follow the suggestions above and select a transformer that is rated for a measurably heavier load than what you'll be pulling from it. If you can keep the transformer from running above appr 60% of capacity, you'll never run into any problems with saturation or thermal induced problems. This also minimizes the potential for any type of dynamic constriction to take place during large swings in current draw / spl levels.
Once the AC is "cleaned" coming out of the mains feeding into the sub-panel, you have to keep RFI from entering the AC feedlines that will power your system. This can be achieved by using twisted pair lines from the sub-panel to the outlets. Using twisted pairs lowers the inductance and reduces the potential for your AC lines to act as a long-wire antenna. After all, filtering out all of the RFI at the sub-panel to have the AC system "re-infected" along the way makes very little sense.
The further the distance from the sub-panel, the heavier the wire that you want to use. 10 gauge should be sufficient for all but the heaviest draws or longest runs, but if you're running very high powered high bias amps into low efficiency low impedance speakers, you might want to try running 8 gauge. This is especially true if the runs are quite long i.e. the other side of the house from the AC sub-panel.
You don't need to run "mega-duty" cables for all of your outlets, but it is preferable to use the same gauge wire for all of the runs if possible. The use of even 12 gauge for all the dedicated runs would probably be a big step up for most every installation if one has been using the standard shared circuitry with other outlets in the house. Ten gauge is not that much more money though and if you're doing it now and want to do it right, use the ten gauge. An electrical contractor might tell you that this is "overkill", and for all practical purposes, it probably is for most systems and installations. For the extra few dollars that you spend using 10 gauge now, you'll know that you'll never have to worry about it ever again. Call it a cheap investment in the "peace of mind" factor : )
As far as how the wiring is routed and what you use to house the wiring, you'll have to check with your local inspector to see what will meet code. If you can use PVC rather than metal conduit, do that. Just don't forget that you'll have to run ground wire too as the twisted pair mentioned above only carries the hot and neutral conductors.
Obviously, the selection of outlets becomes a personal matter, so i'll refrain from making suggestions. Just make sure that the AC lines are connected to the outlets via the screw terminals, NOT the "inserts" in the back of the outlets.
Following this approach, you'll have a pretty phenomenal AC system as compared to what one would normally have using standard house wiring. If one were to stop here, i'm sure that most folks would be very happy with the results. After all, you've now got direct runs of heavy gauge low loss wire that has reduced potential to pick up RFI. All of the AC is filtered to the point of having a noise floor that is at least -120 dB's quieter / cleaner than it was before you started and the outlets offer a more secure grip / better connection to the power cord coming in from each component. The -120 dB figure was based upon using a "good" quality iron core iso transformer. The use of a "very high quality" ultra-isolation transformer can get you into the -140 to -145 dB region. If one chooses to use a toroidal design, you can expect to achieve somewhere around -60 dB's to appr -85 dB's of noise suppression.
Using the above figures as a baseline, let's do some math. The "good" iron core at -120 dB's of noise suppression offers over 4000 times more isolation than a "very good" toroidal at -85 dB's. To take that a step further, the "ultra isolation" transformer offers over 1 million times more isolation than that same toroid and slightly over 500 times more isolation than the "good" -120 dB iron core. Now do you folks know why i say what i do about toroids??? Using even just a "good" iron core iso is WAY more effective than using a "very good" toroidal based iso.
The use of additional filtering at the system or for individual components may also be beneficial, especially if one can filter the entire system at the component level. This keeps the "grunge" generated from one component from feeding back into the other components. This requires either a sophisticated line conditioner with isolated filters for each outlet or multiple individual filters for each component. Personally, i'm running 400 lbs worth of iso transformer at the mains with individual iso's for each individual line level component. Power amps are fed directly from the iso's via the sub panel. I'm very resourceful when it comes to finding pieces and parts at bargain prices, hence my ability to do this and not go broke. Buying large assemblies and then gutting them for individual parts has benefits : )
If one wanted to try something like this without going gonzo, i would try using some type of high quality filter to keep the digital gear electrically separated from the analogue gear. If you can do that, you'll be way, way ahead of the game. The money that you saved by not having to buy "mega dollar" power cords can now be used for component upgrades, to buy more music or see more live events. Sean
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