Damnit I'm not sure if I'm going to be able to upgrade then.
What's the wattage of your power supply? For a card like that you will need at least probably 500W bare minimum.
Your power supply will also need 6 pin PCI-E cables like these:
As for the different brands, just shop around and pay attention to reviews. Try to figure out the one that you think seems to be the best, there's also price to consider. Generally though there isn't a massive amount of difference between brands apart from build quality, fans etc, some might be overclocked and more powerful.
If your power supply isn't up to it you can always upgrade that as well.