I've struggled with the header thing on my car too. I decided to leave the manifolds for now. From what I understand you lose some low end torque with headers. Torque is really where it's at for a street driven car that doesn't see the track much. If you've ever seen the Pure Stock drags, there are guys running 11's with manifolds and bias ply tires! Now, if you do decide to go with headers, I'd buy a set that's got a coating similar to Jet Hot and use locking fasteners.
Todd

Headers are a little louder, but the HP gains are worth it...................Sounds better than a cracked manifold.

I'd say go with the headers and keep your eyes open for an exhaust manifold if you ever decide to put your engine back to stock. I'm not sure what you are running for cam or mods to your engine but it never hurts to let your engine breathe. If you are bringing more air in you need to get rid of it. 
 Don't make the mistake that a lot of people do by getting the biggest primary tube size you can get becuase you don't need it. 1 5/8" to 1 7/8' tube size with be lots. Go with a coated header or get them coated after you make the clearance dents and bends that always seem to be required.
 I have Hooker headers for my car. They may cost a little more but they fit right and they have heavy gauge tube and flanges. I don't know if shory headers are available for your car but I have used them before when ground clearance was an issue and had no problems. Had them on a 350 hp small block with a 600 Holley.
 If you go with headers, before you install them soak the gaskets in water then coat both sides of the gasket with Ultra Copper at the ports. Install them on the engine and run it right away and get the headers as hot as possible then tighten the bolts up. You may want to retart the timing a little bit to get them hot or take it on a hard run. The next time out tighten them up again and you should be good. I had a set on my SB Malibu and after 3 years never had to tighten them again.
 Good luck.

It really depends on what cam you have and what your rear end gears are. Headers will only make a difference in performance from about 3,500 - 4,000 rpm up. If you have a fairly mild stockish type cam and a low numerical rear end (3.00 or lower) you really aren't going to notice a performance difference. You will notice some more noise with an otherwise stock type exhaust but to get the true benefit of the headers, you should either have cut-outs (yeah baby!) or flow through or two chamber mufflers, which you may find to be too noisey while cruising.