We have a Samsung HLP50" DLP and a Yamaha HTR5990 receiver. Any suggestions on upconverting players using HDMI?

You don't state a price range. For upconverting players, the Oppo is a great value. At the higher end, check out the Denon 2930 and 3930. As an alternative, Toshiba makes a nice HD-DVD player for true high def movies for only about $400, and it also upscales regular DVDs at least as well as the Oppo.

$150-$300 range preferred. From what I'm reading on Oppo, I should look at the 970 vs. the 971 or 981 for a DLP (macroblocking issues, etc.)?

For DLP, look at the 970, the 971 and 981 will give you macroblocking issues with your DLP set.