The 6 cores won't benefit gaming. I'd suggest either an fx-4100 or a x4 965 BE (if you want to stick with AMD), or an i3-2120 if you'd like to try intel. Put the extra money towads a 7850, 6970, 570, or 480.
two of those put up numbers like a 6990, and will only run you $320.
It's a refurb'd card, but it comes with a free 1 year warranty, which is better than most new cards, except of course for EVGA who has limited lifetime warranties.
Make sure you have a strong 12v rail for the x2's, though. each card can draw 265w under load. (a 4870x2 is basically 2 4870's, so 2 of those is like running quad 4870's, except you have more useable ram with the x2's)
That Antec you picked has 3 relatively weak 12v rails. Even if you don't use the 4870x2's you should get a PSU with a single strong 12v rail over that. (though @ 264w, each rail could handle a 7000 series card; but only because the series is low power)
I'm typing this on my AMD 955 BE and it's not nearly as pleasurable compared to my 3 year old i7-930 OCed to 3.9GHz. The Intel solution will work better at everything else not just gaming. The AMD route will work fine for gaming but IMHO there just isn't a great upgrade path. AMD is rarely recommended for budget builds - it only gets honorable mentions. Most people don't realize how much Horse Power is needed to give a great gaming experience - far above consoles.