Here is one more to consider.
lenovo IdeaPad Z585 - $499.99 (
Link)
Although this lappy only has an AMD A8, it makes up for it with
TWO GPUs! It has the integrated HD 7640G + Radeon HD 7670M (Discreet Mobile GPU)
GPU Specs (Both support OpenCL 1.2)
HD 7640G iGPU
Bus: PCIe 3.0 X16
RAM: Sys
Clock/ Core - MEM: 496/685 - Sys
Unified Shaders/Texture mapping unit/Render Output unit: 256/16/8
GFLOPS (SP): 253.95
DP Support: Yes
HD 7670M
Bus: PCIe 2.1 X16
RAM: 1024MB GDDR5
Clock - Core/MEM: 600/900
Unified Shaders/Texture mapping unit/Render Output unit: 480/24/8
GFLOPS (SP): 576
DP Support: ???
BTW: My A8-5600K desktop APU (HD 7560D)
can crunch Collatz so this lappy APU should be able to crunch it also.