will it be a great upgrade ... for eg ... boosting performance in games??? and will Pentium D 925 serve as a bottleneck to Ati 5670
It may. But I see no reason to restrict your GPU to the limit of your processor specially if you are planning to upgrade anyway.
Whether you upgrade your processor now or a few months later, you will still be needing that graphics card. So if you buy a pretty weak card to match your processor now, this weak GPU will in turn bottleneck your next processor & you have to buy a new GPU again. So pick your GPU on the basis of your screen resolution. The 7750 800Mhz will be best just a little more expensive than your $100 budget.