The All-in-one Upgrade Kit is a sledgehammer and a cheque book.
I started out with iMacs as my primary computers. A Bondi Blue in 1999, G4 Lampstand 2002 and a G5 20" white slab in 2005. As I progressed thru the cycle I noticed, how little I could upgrade on them.
The Bondi was most upgradable, I modded the CPU (233 to 600 mHz), HDD (4Gb to 80Gb), RAM (96mb to 512mb) and even the GFX card (adding a Voodoo2 8mb to the mezzanine slot).
The G4 I was reduced to RAM and a Airport card as self made mods.
The G5 I could change nothing on, it was already maxed out for RAM and the HDD was huge.
Then my philosophy changed, instead of replacing my Mac every 3 years as Apple obviously wanted me to I bought a base unit 2008 Mac Pro octo and upgraded it myself over the next 5 years. Apple got just £1700 of my cash and the RAM, HDDs, SDD and flashed PC GFX cards all cost me a fraction of what Apple wanted me to pay them to fit. £4000+ worth of Mac workstation for just over 2 grand...
The Mac Pro still runs like a dream today and tears many off the shelf PCs a new one even now.
I went yet another step further this year, I Hackintoshed £1200 worth of PC parts to make a beast 30% faster than the Mac Pro, now they play very well together and satisfy my insatiable lust for power. Just.
If I had stayed with All in ones, Apple or some other vendor would still be rubbing their hands every 3 years.
Once you go Hack you never go back.