Yeah, RAM or Airport. You have to choose b/w the two.
I heard though that some high-speed HDs can actually be faster than the built-in HD. Weird... But if that's the case (and provided that the HDs in question don't cost a billion bucks) then I wouldn't mind getting one.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-