What I've noticed, is that for my 7950 cards best H/s are attained when I set GPU_MAX_ALLOC_PERCENT 80 for all cards (7870,7950,290). Otherwise there is too much video memory allocated for 7950 (some 2.6GB), which results in decreased performance. Optimal value seems to be 1797MB for 7950. Cannot check the values for 7970 as I don't have one.