Everything you have listed will fit in the Antec 300 but its going to be cramped.
If you're interested in WoW and games with the same GPU demands you should get the 5770. The 5770 will use DX11, its cheaper, cooler, and consumes much less power. The 4890 is for games that really need the GPU strength like Crysis (ugh) but its more expensive, larger, limited to DX10.1, produces so much more heat, and consumes more power.
5770 is the way to go, in your situation. You can always Crossfire later.
"The value of the 5770 in particular is clearly not going to be in its performance. Compared to AMD’s 4870, it loses well more than it wins, and if we throw out Far Cry 2, it’s around 10% slower overall. It also spends most of its time losing to NVIDIA’s GTX 260, which unfortunately the 4870 didn’t have so much trouble with. AMD clearly has put themselves in to a hole with memory bandwidth, and the 5770 doesn’t have enough of it to reach the performance it needs to be at.......
So here’s the bottom line for the 5770: Unless you absolutely need to take advantage of the lower power requirements of the 40nm process (e.g. you pay a ton for power) or you strongly believe that DirectX 11 will have a developer adoption rate faster than anything we’ve seen before for DirectX, the 1GB 4870 or GTX 260 is still the way to go"