There's lots of servos out there,but I think you're first upgrade should be a 5 cell pack as mentioned above. The higher 6v nominal voltage of a 5 cell will add torque to whatever servo you use vs. the 4.8v nominal voltage of your 4 rechargeables.
hs-645 would last about 20 minutes in an 1/8 scale. just bite the bullet and pick up a hitec hs-7955 for steering, it has titanium gears and at 6v its 333oz/in. stock throttle servo should be fine for now. get rid of those 4 rechargeable batteries and upgrade to a 5 cell nimh hump pack. you'll love it!