Since the TV offers HDMI, I'd use it - the 990's DVI switching will work just fine with it. All you need is a couple HDMI-to-DVI cables and you're all set. Definitely go that way, as it will let your video signal remain in the digital domain from disc to DLP chip.

Between the two OPPO players (970HD and 981HD), OPPO tends to recommend the 981HD for screens larger than 50". With a DLP, you will probably want to be sure to calibrate it to minimize any macroblock enhancement from the 981HD's faroudja chip, but you'll want to calibrate the video no matter what source you use anyway.

As for the decision between the 981HD and a Toshiba HD-A2 (the entry-level HD-DVD player, which can be had for close to the same price as a 981HD if you shop around a bit), the HD-A2 does a very good job upscaling DVD's - on par with the 981HD. The principle difference is that the HD-A2 supports HD-DVD while the 981HD supports DVD-Audio and SACD. The HD-A2 also lacks multichannel analog output, so you can't get DD+ or TrueHD into the 990 (you'll end up with those formats encoded to high-bitrate DTS instead, which actually works pretty nicely but isn't going to be as good as the original). You've got to upgrade to the HD-XA2 to get multichannel analog output, and that adds to the cost significantly.

With the state of the format war, I'm still a little worried about HD-DVD's long-term viability. Blu-ray has a better chance of succeeding in the long run from a software standpoint, but the player offerings haven't really attracted me yet. Samsung's BD-P1200 may be the best "bang for the buck" among the standalone players, with Panasonic's more expensive player offering the best feature set currently. The PS3 is one of the best Blu-ray players around, but you don't have multichannel analog output or an IR remote (the $25 add-on remote is bluetooth only and the old PS2 remote and IR-to-USB attachment reportedly don't include all of the controls).