I upgraded to double what I had exactly.
2g ram < 4g ram
120g HD < 320g HD
4x SuperDrive < 8x SuperDrive
The processor boost was nice as well 1.83 < 2.4
Biggest improvement though was going from the MacBook video card to the ATI Radeon HD 2600 PRO graphics processor with 256MB of GDDR3 memory
It's actually a bit of a special order iMac.
I'm diggin' it. This thing is juiced and boosted well above what they 2.4 is supposed to be.
Apple sells the 2.4 20" iMac with the ATI Radeon HD 2400 XT graphics processor with 128MB of GDDR3 memory.
This one however has the ATI Radeon HD 2600 PRO graphics processor with 256MB of GDDR3 memory, which is supposed to be in the 2.66 model.
It also has the 320 Gig HD that is supposed to be in the 2.66 as opposed to the 250 gig.
http://www.apple.com/imac/specs/
Combined with the 4 gig of Apple ram, this has turned out to be quite a performer.
Quite pleased.