.
That's a lot of questions. 
You should do the 1MB chip mem mod if you plan on using enhanced or high res screenmodes. 8371 only supports 512K, so you'll need to pull that out (or push it out from the bottom, PLCC chips are a pain-in-the-arse) and replace it with an 8372. After that there's a couple ways, including soldering a few spots on the motherboard, or doubling up the chips in their existing spots, etc. What you decide depends on motherboard version and technical proficiency. Personally I recommend the version of the mod that converts the 512K in the trapdoor slot into chip ram.
You can also add a "Megachip" adapter after that, which would use (typically) an 8375 Agnus, and give you 2MB.
http://en.wikipedia.org/wiki/MOS_Technology_Agnus
http://amiga.resource.cx/exp/megachip
ECS screenmodes ain't much to write home about, but me personally, I like 'em, and I like having my Amiga's as "up-to-date as possible", especially since you can buy those chips all day long for under $20.
You start doing these other upgrades, you might as well look at the Indivision ECS. For under $100 USD this will give you scandoubled output on every screen mode, as well as allow you to use things like the 800x600x16 colors mode. Make sure you upgrade to 1.10 firmware. Obviously you'll want an accelerator to use that screenmode.
Early Startup control ("holding down the two mouse buttons"), you've already got that. That's part of 3.1. You do have an actual 3.1 ROM, right? If not, go buy one. Just do it. It's under $20. Don't listen to these cheapskates who say not to. Then yes, you'll be able to hold down both mouse buttons and switch screenmodes.
Like I said, I like ECS, and feeling like my system is "more modern". But you might want to read what the actual differences are before you start waiving your hat in excitement, about it.
http://en.wikipedia.org/wiki/Amiga_Enhanced_Chip_Set
* Cracks fingers * Here goes with the questions that is coming next. You said that because of my AGNUS itself I can only see 512 KB chip RAM, am I right? Are you saying that in order for my chip ram in my trap door to work as chip ram I have to upgrade agnus REGARDLESS or the attempt is futile? (See how I am going in full details with questions
Baby you have not seeing nothing yet with what is coming next).
1) I need to upgrade AGNUS to get 1 MB CHIP RAM?
2) You said my A500 revision 5 can actually see 2 MB CHIP RAM by having an adapter? Are you saying with this adapter I can get the mega chip agnus and have 2 MB CHIP RAM?
3) Is there an actual 1.50 MB trapdoor RAM upgrade?
4) If the answer is no then is there real point in doing this! My A500 motherboard's already filled with RAM to give 512 KB CHIP RAM in default and there is no space to solder more RAM chips.
5) Do I really, really, really, really need to do soldiering, putting wires left and right to do this upgrade? Remember I have revision 5.
6) If the answer no for soldiering and all I have to do is remove the chips and replace them with these chips I might as will do all of that just for the feeling that I have the choice available, where do I find the megachip adapter? How hard to find them? How expensive to buy them? How difficult in installing them?
7) Already coming by mail the super denise can I install it by itself with my normal OCS Agnus that only sees 512 KB or it will fail and need to do the rest of the upgrades?
I want to buy the 512 RAM trapdoor and place it as chip RAM and not as FAST ram but are you saying that cannot be done with my current agnus until I do the upgrade?
I already purchased the ecs indivisions but in kickstart 3.1 it fails, the graphics becomes jibberish and non functional!! This is why I am sellling it...it is not working with my A500 and because I removed my denise soooo much the pins are bending horribly but it stil works so I ordered another denise in fear this one will be ruined. I just discovered i ordered a super denise and decided to go through this upgrade since I really want 1 MB CHIP RAM and not 512 KB chip RAM
