i know what your on about when it says that minecraft ran out of memory rite? well all you do is in the luncher go to the options and and change the ram size if the game from 1000mg(1g) to 1500mg(1.5g's)thisll stop the game from running out of memory longer but when the game seems to begin to cuse unatral lag(lag that appears for no apearent reason even when you have the game on the lowest of low settings and cat stop it) just press 'esc' then 'save and quit to title' on the game then go back to it...if the game is still laging just save and quit again then re-open tekkit and the lag will go (tip:to stop about 20% of the lag change in the options the auto save to 30 mins if you have it on 2s then your game will last 10 mins aprox so before the 30min mark save and quit and close tekkit and re-open it cus the 30min mark is the lag spike of death).
p.s. sorry for the long reply also hope this helps.