When you purchase the DP also look into either a tune or some type of EM (cobb AP or opensource) so you can flash a "stage 2" map which will give you even more of a power gain over just bolting the part on. Make sure to look into your states emission standards so you know if you can get away with running catless if you are looking that route. Also if you can afford to, get an upipe to do at the same time as your DP as it will be a little easier. It is for safety, not absolutely needed for just the addition of a DP. Like Mosc stated any brand of a bellmouth DP is really the best design to go for. And also as stated a CBE is mostly for noise/look, but you can get it also to "complete" the exhaust side of your car.
PS. Sorry for the long post, hope its understandable and helpful.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX