Joe, you didn't mention what you're looking to accomplish!
If you're happy with your sound, why change? Speakers and subs are still speakers and subs. Amps are still amps. Some 40 year old stuff sounds just as good as today's stuff.
Your amp is also tied into your receiver/pre-amp, so some of the inputs may not be as plug and play (HDMI) but that doesn't mean the signal you get over another connection can't produce amazing sound. I'm betting that your current receiver has an optical digital input. Today's Blu-Rays have a fiber optic digital output. Amazing digital sound, and everything is there for decoding DTS or DolbyDigital (5.1 in your case). So.....why change anything in your sound set up? The only reason I can think of would be a smaller form, like wall-hung slim speakers to aesthetically compliment a wall-hung flatscreen. But they're not "better" and usually they're worse, esp. if you're also using the system for musical listening.
Same story with your rear-projection TV. Other than being **HUGE** those high-end Mitsus (and the 73s were all high-end, at least "Gold-Plus" trim) still rival modern LCD/Plasma monitors for overall picture quality depending on the lighting in the room. If you're in a bright room, you may want to upgrade. Otherwise, you're not gaining much except footprint. And a new 73" flatscreen is still a big chunk of money. Your Mitsu should have at least component video inputs. Again, a new Blu-Ray will have them.
So **if it were me**, I'd first invest in a good Blu-Ray player. Use optic output to your current receiver and component output to your current TV. Many BluRay players have the internet capabilities kachi mentioned, like YouTube, Netflix, etc streaming over your TV. They can be WiFi or ethernet depending on what you have available. You can spend $75 or $475 and I'm not sure you'd see any difference in picture quality.
After the Blu-Ray, address the next weakest link. Is it power? Is it your center channel (the most important speaker in 5.1)? Is it the TV? Is it your sub(s)? Or is it a pretty well balanced system that changing a single component wouldn't really help. Adding $5000 speakers to an old $250 50Wx5 A/V receiver doesn't make sense.
BOSE is overhyped, overrated, and has been for what, 15 years?
So, what are you looking to accomplish?
