I have a 2000 RS, with an automatic transmission. At some point in the future I would like to give it more power, most likely in the form of a turbocharger.

What would have to be done to the transmission (automatic) to handle this power reliably? I would expect it to get somewhere around 210-250hp and would think the stock transmission could not last more then a few thousand miles like this, but I could be wrong.

Any ideas?

If you do a search you will find that there have been no problems with turboing/supercharging Automatics. Mjos has one on his 99RS auto and Kevin Thomas has one on his 2.2L OBS auto with a 50 shot of NOS.
Rich (fellow auto owner and soon to be "snailing" away in his 2.2L)
As Tyrmelter says it's all about the "Snail"!

The 4EAT does ok in the SVX (94+ at least) with about 230hp at the crank, and the situation with the lighter impreza should be even better.

The 4EAT really doesn't seem to be weak... after all, it works just fine in 300ZX's.

However, the 4EAT's subaru put in the pre 94 SVX's did have some heat problems. If you want some piece of mind, then putting an auxillary cooler unit in is pretty cheap.

You're probibly fine without it however.