If the charger is at least 800ma. then that is about as much as you need. The max charge rate is about 800 ma. The battery is 2100ma. So figure (roughly) 2.5 hour for a full charge.
BUT, if the phone thinks it is plugged into a PC USB port, it pulls about 300ma. Figure more like 7 hours to fully charge. If the connector is starting to fail, if it is the wrong type of cable, or the wrong type of charger, it will charge at the slower PC USB rate. If you need a fast charge, then it needs to be a charger / cable combo with the correct wiring. Install 'battery monitor widget'. It will identify the charger type (AC versus PC USB) and track the current to let you know if a cable is failing, or the after market charger is a low rate charger. There is no point in a charger much over 800ma because the phone can't use it. But the charger and cable MUST identify it as the higher rate.
If the phone is pulling a lot of power also, like the screen on or heavy data usage, then less power will get to the battery. I have also seen at least one cable that had such small wire in it that it couldn't supply the current, but that is rare.
BTW, I use a low rate charger for overnight charging anyway just to keep the phone cooler. I only use a high rate charger during the day when I need a quick charge.