With GDDR5 RAM if your Band Width is 80Gbyte sec your FPS in the alpha right now will be around 30 sitting on the runway and 40-47 from 1000-5000ft.
In the alpha graphics as is for testing, GDDR5 RAM and a Band Width of 80 or better will return reasonable performance. This means data paths of 128bit and higher. Even if you have GDDR5 RAM and 1G, if your data path is only 64bit, don't expect a lot out of that card at the moment.
GDDR3 is 2x slower than GDDR5 even if your Band Width shows 80, looking at the GTX 9800 as the minimum card standard running the game in 512 Mode.
512 Mode
9800GTX
512 GDDR3
256bit
Band Width 70.4 Gbyte sec. <---you still have to turn everything down.
1024 Mode
HD6770
1G GDDR5
128bit
Band Width 80.0 Gbyte sec. <---30 fps sitting on the runway with defaults on.
I have to believe since this game code is GPU intensive that Hitech is aware of the performance differences players will bring to the arena with GDDR3 and GDDR5 Speed\Band Width. Just like in AH1-AH2 the CPU and video card performance differences. He planned for the differences to allow a broad spectrum of PCs to play his game.