I would love to get that, but arent they REALLY expensive? Is it worth it? Plus I dont think they sell them in the US do they? know anywhere I can get one? 
To get the best picture out of most of those systems you have to buy RGB SCART cables and the XRGB together. You would be able to use either the VGA or DVI port on the back of the XRGB3 to connect to your TV via VGA or DVI to HDMI, VGA is actually the better option since it's an analog connection.
There's even a better one named the XRGB- Mini or Framemaster that just came out last year. Here's the review of it with lots of pictures:
http://retrogaming.hazard-city.de/framemeister.htmlTo answer your question, if you are willing to spend the money for the best picture out of those systems then yes it's really worth it, the difference between composite and RGB is night and day especially when it comes to modern HDTV's. See for yourself, Emulator vs RGB vs Composite.
http://www.chrismcovell.com/gotRGB/screenshots.htmlAnd if you decide to buy an XRGB they are avaible on eBay for around $400-$500.
Ask for composite up scaling from the NES and S-Video or composite upscaling from the 2600, I couldn't tell you, but if you wanted something to do it, the XRGB would be the upscaler.
Oh btw, here's the list of systems you posted and what they are capable of outputting:
-NES : Composite (Can be heavily modified for RGB but it's very expensive and not the original PPU)
-SNES RGB Native
-moded Atari 2600 Composite or S-Video
-Sega Genesis RGB Native
-Turbo Duo RGB Native (Has To Be Modified to be Routed to the A/V Out Port)
-Neo Geo RGB Native
-Sega Saturn RGB Native
-PS2 RGB Native or YUV (Component Video) This one can go either way since you can switch the video signals on the PS2 menu. Component would be the easiest but RGB would be purest signal.
-Dreamcast VGA Native (Get A Dreamcast VGA Box, as long as your TV can display 640x480 then your set, if not just plug it up to the upscaller)
