Suspension components or tools.

I'd especially recommend the tools if you plan to do the swap yourself. Get a set of good screwdrivers, metric wrenches, metric sockets, and maybe a dremel. I recommend Craftsman tools not only because they are good quality for the money, but they also have a free replacement guarantee if you break a tool (not likely).

Suspension components are also an extremely good idea. You can get a set of sway bars for under $250, and most people say sway bars are more of an improvement in handling than shock & springs. Tower strut bars or lower tie bars are also good investments. Just make sure you get decent/good quality (none of that e-bay stuff). Like quikstep said, suspension components stay the same even if you swap, so they are always a good idea.

Bushings are also a good idea. If you get really concerned with handling, buy the Energy Suspension Hyper-flex bushings kit. They tighten up the entire suspension and make it feel like a new car. The hyper-flex kit includes shifter bushings, which I would recommend buying separately if you don't want the full bushings kit. If you are going to install these bushings yourself, be prepared because they are a ROYAL PAIN IN THE A$$.

Unless the motor you swap in uses different motor mounts, the Energy Suspension motor mount inserts are also a great, cheap mod.