whats the limit on batt cells?

im still using 2 of 6cell nimh packs 4600mah on my savage.i want to buy the 7pack or maybe more and run 2 of the 5000mah 7cells.will that be ok with the quark125 and 8xl motor?i cant afford to buy li-po batts/charger so its not an option..any suggestions?
what will be the difference between the 6cell 5000mah and 7cell 5000mah?more power or more run time?