120GB or 20GB?

We're thinking about finally buting a X360 at the end of the September,* and are debating between the Console ($350, 20GB) and the Elite Console ($450, 120GB).
As far as we're concerned the $100 difference would be primarily for the 100GB difference (though I also prefer the black paint) - we don't use X-Box Live at all.
But I don't know how fast the 360 eats memory. Is 20 Gigs enough for the casual player? Or is the 120 Gigs what we'd really need?
Also, any tips on keeping it from overheating?
Thanks
*I have no interest in the PS3 or the Wii, so don't bother trying to sell me on one of those; we're buying the 360 mostly to play Oblivion and Halo 3.
As far as we're concerned the $100 difference would be primarily for the 100GB difference (though I also prefer the black paint) - we don't use X-Box Live at all.
But I don't know how fast the 360 eats memory. Is 20 Gigs enough for the casual player? Or is the 120 Gigs what we'd really need?
Also, any tips on keeping it from overheating?
Thanks
*I have no interest in the PS3 or the Wii, so don't bother trying to sell me on one of those; we're buying the 360 mostly to play Oblivion and Halo 3.
