BOV is on the cold side and uses manifold vacuum, I use the full vacuum source on the bottom front of the carb (4150 style carb at least), what happens is when you close the throttle it pulls a vacuum and that pulls the BOV open venting the pressure backed up in the intake piping when the throttle is slammed shut, it keeps the turbo wheels/axles from being super stressed when it spikes pressure in the "closed" system at that point.
Wastegate goes to boost source, any source of boost pressure. The spring in the wastegate keeps it closed to a certain boost pressure, once that boost pressure is hit the wastegate starts opening venting exhaust pressure so it isnt spinning the turbo any faster than needed to get to what boost pressure you want.
These are better explanations
http://en.wikipedia.org/wiki/Blowoff_valve http://en.wikipedia.org/wiki/WastegateFor the exhaust going into the turbo, you can keep it split into the turbine housing or join them and have them go in together, the S400 is "twin scroll" meaning it has the two holes on the turbine housing inlet instead of just one. If you keep it split, you'll need two wastegates, one on each side. If you join them then you can run one wastegate, a little bigger, that vents them both together. I kept mine split and have to run two small wastegates but I dont think it really does much, the turbo spools so fast it isnt needed and alot of people say it doesnt really do anything. Supposedly keeping them split makes the turbo spool slightly faster.
Wastegates are expensive and there are lots of different opinions on them. Them malfunctioning can spike your boost and burn up your engine, so alot of people wont use the Chinese knockoffs. Some good single ones to use if using only one would be JGS 50mm, Tial 44mm, Precision 46mm. The Precision 46mm is by far the cheapest but I think they might be chinese made, but Precision is a big company so I cant imagine they are bad. I have a Precision 39mm on my Monza, and another chinese knockoff, they both work, they should be matching wastegates but they work together fine. The JGS and Precision come with multiple springs to use for different boost settings, the more expensive Tial only comes with one for some reason. Tial is kind of known as the best of all of them but they all seem to work. You might be able to get away with a smaller cheaper gate like a JGS 40mm. If you put it on and it doesnt control your boost low enough for your liking, like lets say you can only go down to 10psi with it and you want to run 6psi(dont know why!) then you can always add a second gate. I personally wouldnt care about running below 10psi or so on my setup, so a single JGS 40mm would probably be enough, it just isnt as much fun once you've felt the insanity of 10psi and more.
Sorry for the long post, you'll get 100 different opinons on what wastegate to use, so take my rambling with a grain of salt.