LockON is CPU bottlenecked if you already have a decent GPU, a new one will not benefit you that much. (As for other GPU limited games, it would really benefit you, just not LockOn)
Remember, all the above is based on you having a decent GPU already, X600 isn't extremely powerful, so a new card should give somewhat of an improvement, at least the capaility to run more effects on and AA or AF...
That is.. If your CPU is fast enough to feed the new GPU, it's all a vicious circle ;)