The Panasonic plasmas have pretty good scalers. How does SD from your cable/sat look? Many standalone Blu-Ray players (e.g. Panasonic's) have crappy scalers. The Samsungs with the Reon's are good, but they are less than great Blu-Ray players.
Consider
#1) One of the Anchor Bay Toshiba HD DVD players (e.g., HD-A20, HD-A30, HD-A35) for about US$120-200 on E-Bay,
#2) The Reon based players like the HD-XA2. However, they are still expensive.
#3) The Oppos. The Oppo players (not that POS macroblocking 981H--keep it away from your plasma) also play SACD and DVD-A.
#4) The PS3. It won't be in league with the first three options I listed. However, it will also play Blu-Ray discs, WMV, DivX and AVCHD. Some models even play SACD's.
#5) One of the cheap 2nd/3rd gen 1080i Toshiba HD DVD players. They are okay upscalers and can be found for less than $100 US. However, I would take one of the above options if I were starting out.
#6) One of the 1st gen 1080i Toshiba HD DVD players. They upscale well, but they are slooooow.
#7) A regular upscaling DVD player -- either higher end like Denon or a run of the mill Philips, LG, etc. I would do this one as a last resort. Your display probably has a better upscaler.
I forgot to mention upscaling receivers and standalone ($$$) scalers and video processors. However, most of those would likely exceed your budget. The same goes for the combo HD DVD/Blu-Ray players on the market. The Samsung has the Reon processor, but still has many playback issues according to the latest reports on AVS.
I will ask at the very least that your calibrate your display or have it calibrated to get the most out of your viewing experience.
Good luck!