I have my 507 set up with the Key Digital HD Xplorer card now and it sure looks great!
I am experiencing the same challenges as Josh, so I'm currently running the card on 1024x768. The plasma does accept 1280x800, but the HD Xplorer only outputs either XGA (1024x768) or WXGA (1280x768).
My sources are Verizon FIOS HD TV over HDMI, a BOSE DVD player (not HD) over S-Video and a Desktop computer connected directly into the plasma's internal DVI port.
Even though the panel is running at 1024x768, it does look very nice. I was surprised by how sharp and vivid the SD picture from a normal DVD on the BOSE system looked on the screen. I am using the PC for digital pictures and that simply looks awesome. I have an nVidia graphics card on the PC and with the latest nVidia software I am running the 1360x768 resolution. This is plug'n'play and looks so sharp and natural that the people in my pictures seem real! I tried at first to connect the PC to the Xplorer card, but that was pointless. Since the card doesn't support the native resolution yet (firmware expected beginning of December) it just downscaled the picture to 1024x768. It still looked very good, I am impressed by how well it scales the different resolutions, but I was looking for that true 1365x768 resolution so I plugged it into the DVI port instead.
The Xplorer card gives you complete control of most anything, I really like that. You can adjust anything and every setting you make is specific to that port, so every source can be customized.
The only problem I have is that I now have 4 remotes (my wife really likes that...); FIOS, BOSE, 507 and Xplorer... If any of you guys out there have a recommendation for a good (but not too expensive) programmable remote, I'm all ears.