No the 5000 series isn't coming out for a little while. No use in waiting.
Crossfire is ATI's version of SLI. It means that you can put two of the same cards in your computer and link them together to improve performance
Voltage tweaking is for overclocking your video card. Higher voltages can allow you to get a higher stable overclock. Too much voltage though and you can burn out your card and increasing your voltage also increases the heat output of the cards. I take it you won't be overclocking though so don't worry about it.
You should get this one instead of the Asus though. It is cheaper and will work just as well. XFX also has better customer support and a great warranty.
Newegg.ca - XFX HD-489A-ZDFC Radeon HD 4890 1GB 256-bit DDR5 PCI Express 2.0 x16 HDCP Ready Video Card - Desktop Graphics / Video Cards