t400 + advanced dock + gpu 6570 ram problem

[m41lun0]

hi i have a t400 my specs are

t400
cpu: core 2 duo p8600 2.4 ghz
gpu without dock is onboard only intel gma 4500mhd
i am using the sapphire radeon 6570 1gb when i have my t400 on the dock it wont boot up unless theres no more than 2 gb ram install if theres more it just goes into a loop and never gets to bios... it works fine with 2 gb ram but its really laggy/slow when i only have that much i am using windows 7 64 bit... any suggestions?

p.s. i do not have the switchable graphics onboard...