The system im looking at is 1.42ghz 512mb ram and comes with an 80GB HDD
This is good. Do not settle for anything less than the 1.42gHz. Too many of the USB 2.0 eMacs with 1.0 and 1.25Ghz logic boards are affected by a near-fatal logic board issues that in no longer under warranty.
The other advantage is the ATI Radeon 9600 chipset; it has Core Image support--very useful for Leopard (OS 10.5).
Remember the max RAM with today's modules on that model is 2G (2 x 1G modules), but do not try to get these larger modules "just anywhere." Even Crucial doesn't list these; they never waver from the max RAM shown on Apple specs sheets that, for the USB 2.0 eMacs, are badly outdated.
I bought this 1G module of our eMac 1.42G about a year ago:
http://eshop.macsales.com/item/Other%20World%20Computing/2700DDR1024/
It's working perfrectly.
Our 1.42 eMac has been a very stable computer, and is about the only "G" series computer in the house that doesn't struggle too much with most online veideo. However, remember that a lot of video content on the web is no longer optimized for G4 processors, so some vids may display poorly. You can't upgrade anything to fix that.
Hard drives: The 2Gen eMac is not a simply disassembly. Actually, it's the
reassembly that usually bites you. There are a lot of reports here of people damaging the power switch assembly during reassembly. Unless you consider yourself an above-average computer tech, you may want to consider using external drives. You can put a bunch of FireWire externals (functionallly faster than USB 2.0) and bootable) and not have to crack the case.