#1: The type of ram can vary. It seems that the tolerance to ram gets tighter the bigger size of stick you use.
IE: almost any 128 seems to work, whereas only specific 512 modules work.
So your answer to #1 can vary depending on how much ram you plan to use.
#2 I personally own a G4 version, but I am told that when you compare a G3 and G4 together, the G3 is actually faster at the same mhz - UNLESS you use the G4's Altivec. When the G4 uses Altivec there is a very marked speed increase.
#3 I would personally just use the voodoo3 right now. You already have it, and then your next video card buy can be to something that has great support in AmigaOS, not to mention the fact that video cards have a habit of plummeting in price. There is absolutely no point in buying something now when there are no games out to use the power of the thing anyway.... not to mention possible support/driver issues of the latest greatest video card.
I have used an AGP voodoo3 2000 and a PCI voodoo3 3000 in my A1 running OS4, so it's already supported. I really think that's your best bet now... use the voodoo now and wait.
#4 If you're weary of buying from softhut, look on Amiga.com for other AmigaOne dealers. I bought mine from livewiresystems.com, and have got great service from them....
#5 Debian comes with a quickstart guide, and the CDs you need to install it. The installation is simple, and you'd have it done in an hour. It's the fun *after* that point that is trying ;-)
And finally.... if you're interested in running AmigaOS 4.x, the mention of "Pegasos" has zero credibility. Pegasos is very unlikely to ever have AmigaOS running on it legally.