Small, blue plug? VGA.
Think of your processor upgrade this way: you'll be getting an extra 50MHz on top of your existing 450MHz (times two, of course, because of dual processors). That's approximately an 11% increase in speed
at best (simple math: 50MHz / 450MHz). Is it worth it? I don't know -- only you can answer that. How much are you paying for the dual 500MHz G4 upgrade?
You can look here for upgrades and prices available for your machine, then make that determination yourself:
http://eshop.macsales.com/shop/accelerators/
(the site also lists video and RAM upgrades as well)
If you're wondering how well a particular processor performs in your machine, or the compatibility of a specific video card, visit this site to read real-life reviews of people who upgraded their machine:
http://www.xlr8yourmac.com
(click "database" then choose either CPU or video)
A DVI connection doesn't necessarily mean "LCD monitor"... it could also mean flat-panel TV, a projector, or any other video device that supports a "digital" connection ("DVI" = "Digital Video Interface").
An ADC connection is an Apple invention that carries both a digital video signal as well as power, negating the need to use two cables (video cable and power cable) to run a monitor -- one cable carried both the video signal and the power. It was relatively short-lived, and is no longer in use anywhere. "ADC" = "Apple Display Connection".
You can also look here for deals on used Macs:
http://www.lowendmac.com
(click "Deals")
There are video card upgrades for your machine that run up to $200, and for $200, you could possibly get an even better machine that will go much farther than yours.
In the end, though, no one but yourself can answer the question, "Is it worth it to do X?" because only you know how much money you want to spend. Only you know how long you want your machine to last. Only you know what you use your machine for.
No pressure, but it's all up to you, bud...

You gotta do the research yourself, and you gotta spend time comparing different upgrades and prices, and figuring out your own budget. We can tell you what your machine is compatible with, and possibly offer some opinions, but one man's opinion may not match another's. Some may tell you your machine is old and outdated and not to spend any money on it; others (like me) may tell you to max it out (because I like collecting old Macs) and keep it as long as possible.
If you have technical questions about the upgrades, we're always here to help, but questions like "Should I...?" and "Is it worth it to...?" and "What's the best...?" are subjective and can only be answered by yourself.