What you are hoping to get and what your card is capable of running are two entirely different things.
Riva Tuner is telling you that the card will crash at the higher clock frequencies you want to use.
The test it runs is returning errors at those settings and so it wont let you apply them.
You could try NVTweak as an alternative to Riva Tuner click here
You have to recognise that there are limits beyond which your card wont go.......and that limit will be different for each card.
It's also worth pointing out that even with a substantial overclock an "le" model card will still be a poor performer.
If you can achieve a stable overclock of around 15-20%,as you have ,you are doing very well with the standard cooling and voltage.
A lot depends on what you want to achieve with the overclock.