price range is from $420ish to $500ish and I have no idea which to get.
Asus is a good brand, and, though I don't have personal experience with them, many people say that XFX is also very good. The difference in pricing that you're seeing is because some of those cards are either factory overclocked or are the Eyefinity 6 edition (6 mini DisplayPorts vs. 2 DVI+1 mini DP+1 HDMI) of the card. Make sure you know which version you're looking at before you buy.
OK, so I'll give you a few different features to keep an eye out for when selecting 5870's:
Amount of RAM: Most of them have 1gb, some have 2gb.
Clock Speeds: Some of the cards are clocked beyond the reference speeds, meaning that you get more bang out of the same card and the warranty on the card covers the factory overclock. The clocks on the 5870 can range from 850 all the way to 950, which is a pretty substantial overclock and uses more electricity and produces a lot more heat and stress on the GPU.
Eyefinity: Some of the cards are "Eyefinity" editions, which means they do not have the standard DVI ports. These cards can support up to 6 monitors at once and are meant for true super enthusiasts. Generally, most people aren't going to need an eyefinity card unless they have an exceptional multi-monitor display.
Cooler: Most of the cards you'll see with the "reference" cooler, which is the same cooler as ATI's standard 5870. Some of the cards however, like the MSI brand cards and some others, have aftermarket coolers on them to increase cooling and airflow. Usually these go hand in hand with an overclock already on the card, which is why the aftermarket cooler is on their in the first place, to compensate for the additional heat generated.
Size: In extreme cases, some of the cards are even different sizes, although this is not usually the case. Some of the cards can very in length, so double check the dimensions in the specifications, as well as the power and pin requirements, to make sure that it'll fit within your case and be compatible with your PSU.
When a card lists the memory clock for a video card, you have to take into account that you multiply it's speed. Example: Video card has 1200 mhz GDDR5, it's effective speed is 4800 MHZ. GDDR5 has an effective multiplier of 4x. So your second card has an effective clock of 5000 MHZ with a core clock of 875.
The second card is faster than the first, that's why it's the "XXX" edition from XFX.
Sorry, a quick update: The advantage to buying a card with an overclock ALREADY on it is because that overclock from the company (XFX, Sapphire, EVGA, etc) is covered under their warranty. If you took a stock card and overclocked it, even to match one of their own overclocked cards, you void your warranty right there.