Sorry to say, but you're in for a lot of grief if you're "not real familiar with this swap"...
For starters, this isn't the right site for this kind of mod- take a look at www.rs25.com
For one thing, there is a spreadsheet comparing the cost of a STi eng swap into GC8 Impreza vs turboing the existing 2.5. (Basically, you're looking at around $10,000 in addition to the cost of the car... and a whole lotta work)
Many of the above posts point out the problems involved- you will have a "Frankenstein" car, no Subaru dealer will touch it.
Your biggest problems will be wiring harnesses (hint: you will have to take two harns and make one) and engine management- what ECU are you going to use??? A JDM STi in your US-spec RS? A US RS ECU certainly ain't gonna work...
Don't get me wrong- if you *really* know your stuff, a JDM STi engine in a GC8 Impreza would be one FAaaaAAst and nimble mo-fo, but if you DON'T know what you're doing, you're gonna have a very expensive car that doesn't run well, doesn't pass emissions, and will require more work than it's worth...
So, if this isn't the kind of stuff you're going to do yourself, you'd better have a local shop that's extremely well-versed in both engine swaps and Subarus.