This ultimately depends on the quality of the optics in the lens itself, which is going to vary from lens to lens. SMC Takumar lenses are more capable than some of the modern lenses offered today, so to really know for sure, you'd have to do a side-by-side comparison.
The price of new 500mm glass could prove to be prohibitive, and you may not find that much difference between it and the Takumar. There is a reason why Canon users, who have access to L-glass, buy adapters or modify their cameras to fit Takumar lenses. My personal opinion is that 14 megapixels won't out-resolve your lens. because the finest grain films available at the time of the release of you Takumar could capture upwards of 20 megapixels of information, properly exposed of course.
The only way to know for sure is rent a DA* 300 or some FA glass and compare it to a SMC Tak 300/4 or similar.
EDIT: On a side note, I completely agree with the first response by Graphic. The amount of time spent pixel-peeping for an answer is directly proportionate to the amount of time NOT spent out there taking pictures and bettering your skills. You have to ask yourself what you want out of your pictures. Don't forget that even the best lenses used by some DSLR users have their images edited in Photoshop for contrast, sharpness, etc. It's best not to look at your shots when you get home and compare them to professional pictures you bought and hung on your wall. You never know how the professional got there in the first place.