welcome to NASIOC.
Turbo's are an option but cost lots of money. Others will say to just do a WRX engine swap, but again this costs lots of money. If you couldn't afford an 03 WRX you probably can't afford to do a swap or turbo upgrade. See this post: http://forums.nasioc.com/forums/showthread.php?t=717853
You will have other problems too, like needing bigger better brakes and because of this new rims and tires, not having a Limited Slip rear Differential although it doesn't prevent turboing...
If you asked before you bought I would have suggested looking at an Outback 2002+ H6. Sedan or wagon. The H6 gives significantly more power than the H4 off the line, about 2 seconds with the 4EAT, but is still about 2 seconds slower than a 4EAT WRX 0-60MPH. But then again it too costs more.
The H6 eninge is another upgrade option for the Legacy but also expensive.
You might like to look at a High Stall Torque Converter though. It will help the acceleration of your Legacy and only costs about $500 US.
PS Sedan or Wagon?