There hasn't been a firmware update in years for the 520. I think DLink has stopped all support for this model. I'm not sure about the 750, but I just picked up a WDTV Live from Costco, and it's great. It plays any file I've thrown at it, the fast forward and rewind work, not perfectly, but much better than the 520. Also the firmware is available to hack, and there's programmers out there that have created some good features that weren't provided in the original firmware.
I still use my 520, but I don't expect any further enhancements or improvements by DLink.