the only problem was that after I removed 5870, 6950 started to freeze win7 x64 when I play shockware flash videoclips(with or w/o hardware acceleration)
different-generations of GPU's mayb use different kernel-space drivers, sometims incompatible w/each other. esp tru for non-WHQL-certified drivers[usually cleaned from this to get "WHQL" logo], aka "hotfix", "beta", "RC" and etc.