sure 400 whp is not a problem, built engine (pistons, rods, valves, cams, springs, harder loose bearings), port and polished heads, twin turbo front mounted, intercooler, real exhaust system, sheet metal intake, bigger T/B, heck as far as I know these NA FI cars have a hard time dealing with boost and anytype of open / close loop transitions, so I'd toss the factory FI completely and do a stand alone with new pump, line and rails and injectors maybe even twin injectors cause 1000cc injectors suck at idle. Figure the trans would need to be built and have a new brain, toss in a stall, the driveshaft is hopeless need a new one, the rear end is hopeless so you would need the 216mm with LSD and some 3.92, shafts hubs, might as well toss on the R/T rear brakes while your back there, assume the ABS won't work now with the stand alone and the bigger brakes so toss that out. $20 to 25k should just about cover it. On race day I can see 500 WHP before the walls balloon out on the block. Would be fun. Cost overruns and time overruns, the car will be off the road 1 year or more.
If I went to boost, the limit as Matrix has said is 300 whp before the trans goes, and not much more before the pistons go.
19K is good for a trade on a R/T, I figure mine is worth 16K max and no DWB in 2011, or I would think about it.
So there is NO cheap way to hotrod a V6 to stock hemi levels and to go above SRT levels it is going to be expensive, and it will take time and you will find limits. It would be fun but it would make NO economic sense.
The Pentastar is a little different as at least you can get the 216mm rear and brakes from the factory.