from what I hear, and soon to try, if you have a 2bbl 3.8, then even a junk yard 4bbl manifold from 3.8-4.1 will improve performance, if not aftermarket intakes are avail...headers are a good idea. If your really into it there are some good cams available if you search long enough. If your car is anything like my Monte, rear gears are a litlle steep (great for highway, but the drive in between is little slow) you could try changing those for better acceleration, and if you have a bigger cam, torque converter might be required...if everything else fails, there is always the GN engine, sometimes hard to find, but well worth the wait for the turbo, and better crank.
A lot of the same tricks for V-8's apply for the 3.8, (like higher compression, intake, cam, etc) but availability is sometime an issue.