Regular DVD will look as good as the transfer allows. If it's a well authored disc, using good source - it can "approach" BD quality. Obviously, it won't look as detailed as BD is capable of, however, it will be excellent.
As the poster above stated, displayed on a Kuro, you will be extremely pleased. However, be aware, as much as love my 141... the real heavy lifting is done in the Oppo. Any quality display will give fairly similar results, with the exception of perhaps black level & detail. But, you'll need to properly calibrate your panel to eek out that last little bit of performance.
BD played on the Oppo, displayed on the Kuro, on the other hand = as close to perfection as you can get
SD will be a notch or two down from there. Once you go BD... you probably will not buy SD's any longer, unless they are not available in BD format.
One big complaint I had with DVD is the sound quality. Often the background music is so loud that I can't hear the dialogue. Will the Oppo improve this too?
Sadly, unless you have hit a string of poorly audio mastered discs, this is most likely your environment and/or equipment (not the DVD player). Does your AVR allow setting trim levels, distance, crossover, etc? If there is an Audyssey-like auto-calibration function - have you ran it? It sounds (very broad guess) like your center channel is lower, relative to the others. This would be correctable, if your AVR has the adjustments, by calirating the trim levels on all channels.
If you use the analog outputs of the Oppo, you can trim it in the player. So, that will help, if your AVR doesn't have those adjustments.
Take a peek here for more info on the player. http://www.avsforum.com/avs-vb/showthread.php?t=1124287