My Bose is a couple years old and has composite or S-video only. It doesn't have componenet video. It does have optical, which I plan on using from the HD box.
I think the newer Bose models may have HDMI/composite.
because of the limited connectivity, you'll want to be thinking about replacing the bose with something that has hdmi and/or component switching capability somewhere down the line... not sure what the hd box from the cable company will do, but in a perfect world, you'll route everything thru the receiver... that way, one button push will switch both the audio and video inputs.
wrt the hdmi port, there have been reports of incompatibility issues in certain situations, fwiw... as etr pointed out, component is very good, and much better than the y/c s-video cable.
therefore, your weak spot with the present hookup is the dvd player->bose->tv... two analog connections in a row like that are going to be lossy, but it appears to be your best option.
for a cheap bose replacement: join clubonkyo.com, then go to shoponkyo.com to buy a refurbed TX-SR504 7.1 channel receiver... you'll get component switching for ~$162 or less, then replace that when you get the hd-dvd player, because you'll want hdmi switching capability.
now to really muddy the water

look at OTA(over the air) tuners and antenna instead of paying the outrageous hd cable bill... ota hd has better picture quality than you can get from cable, because it's not compressed like cable is.
www.antennaweb.org to check your local tv station reception.
i have this samsung hd tuner, it even works with the component inputs on my old ntsc tv, no need for an hd tv:
http://www.avsforum.com/avs-vb/showthread.php?t=728392&page=1&pp=30so the dvr to wait for is the hd-dvr with that 5th generation or better chipset, so you can record both ota and hd-cable signals.