Good power supply is not enough. USB ports on RaspberryPi are behind polyfuses which limits current that can drawn from it to about 140mA (in practice, it should be even smaller). So no matter how good your power supply is, if your USB device wants more than say 120mA of power, it will fail. Note that USB specification says that enumerated device can take up to 500mA so some devices are designed to take much more power than RaspberryPi could give them.
What is even worse, the more power you are trying to take from USB, the bigger resistance is on polyfuse which means there is bigger voltage drop. This may be another problems for USB devices. You normally gets less than 5V on RPi with cheap power supplies anyway so dropping it even more may make it out of spec.
Also bear in mind that your device may be working at first but stop working after couple of seconds/minutes or at bigger load or when there is a hot day. Power consumption of many USB devices is not static and can change depending on many circumstances.
Also, I would say that 750mA power supply is not much. It could be OK for normal workload but not when you connect some power hungry devices. And WIFI card can take a lot of power. Note that maximal current that power supply can provide is not the only critical parameter. In most cheap power supplies, the more power you get, the lower voltage is. And if it drops below 4.75, you can have problems with your RapsberryPi.
So while some people got some Wifi USB cards to work on RapsberryPi without powered HUB (but probably having better power supply than you), it's not recommended setup. The recommendation is to use RPi USB ports only for mouse/keyboard and attaching everything more power hungry using powered HUB.