What will be the consequence of having a Tahor heart?
Rashi: I will not sin again.
Radak: From now and onwards my intent will be good. You will help me, and support my heart and Ru'ach to be pure and proper, so the Yetzer ha'Ra will not rule over me again - one who comes to purify himself, [from Shamayim] they help him (Shabbos 104a). This is a new creation!
Mesilas Yesharim (Perek 16): Taharah is not leaving place for the Yetzer ha'Ra in one's deeds. All his deeds are based on Chachmah and fear, and not sin and desire.
Malbim: It will not lean towards or desire evil.
Why did David request separately for the heart and Ru'ach?
Malbim: Chet comes due to desire, based on physicality. Avon is crookedness of intellect; it comes via bad images that the Ru'ach puts on the heart. One who does not do Chet has a pure heart; one who does not do Avon has a proper Ru'ach. The heart is the physical, external Kli in which dwells the Ru'ach, the inner power of the Nefesh. A proper Ru'ach puts only true and good images on the Nefesh. The Lev's nature is to be Tamei; physicality leans towards desire. A new creation is needed so that it will not do so. The Ru'ach's nature is to be proper, until man makes it crooked. He asks only that Hashem restore it to its initial state.
