Update on GT 640 PPD numbers after overclocking, and an additional note on the MSI flavor of the card I ended up with.
First, the numbers:
Estimated PPD: 10301
Estimated TPF: 19 min 42 sec
Note: This is for the MSI GT 640 1GB
I got, which I do not recommend knowing what I know now, considering there are other cards available which are less bandwidth-limited and also have rebates.
Here's the deal with this card. The Newegg specs only listed 128-bit DDR3 but no effective clock speed, which in the future I am probably going to take to be a red flag. Reference design GT 640 cards run the ram at 891MHz (1782 effective). GPU-Z let me know that this one came stock at 667 (1333 effective). This made the bandwidth-starved nature of this card even worse -
The good news: I was able to goose the RAM up to 800MHz, which is at least closer to the reference design, and also boosted the core clock from 900MHz to 1058 (the same core clock as the GTX 650 non-Ti). It is with those clocks that I got the updated PPD indicated above. I could try and push the RAM more, but I'm not sure how well the memory is cooled on it, and I'm pretty happy with where I ended up, all things considered. Went from 6K PPD with a poorly cooled GT 430 to 10K with a Kepler card that has a better 2 slot cooler. For now, I'm calling it a win, even if it wasn't as good a value as it could've been.
*waits for his rebate*