www.240sxforums.com <-- good to start off with. They catagorize chassis and motor swaps in different forums so its well organized and easy to pick up.
www.zilvia.net <-- well rounded forum. IMO its the Honda-Tech of 240s. U'll get flamed if u mention you switched from a Civic.
www.freshalloy.com <-- good technical info concerning high HP build up.
SR is the most popular swap. It bolts right in. Slight wiring modification is required
CA is kind of like an SR but kind of like a mini RB. Its not too common but swap is similar to SR.
KA-T setup is the next most common and will probably get more popular soon from the power it can put out and cost. No motor swapping, just bolt on turbo.
RB motors are considered the Godzilla swap cuz.. well ya know. It requires custom mounts and drive shaft. There are places that make and sell them. I dont know too much about wiring for RBs but it has been done many many times.
LSx seems to be the new Godzilla swap that will be more common within the next year or so. It requires custom mounts, driveshaft, and headers. Wiring work is required but easy supposedly.