Downgrading iPhone 3G to 3.1.3 from 4.0?

[mmmmmmmmmBaMmmmmmmmmm]

Im currently running 4.0 I accidentally upgraded to 4.0.1 and downgraded back to 4.0 and the system resource usage is horrible...
So im downgrading to 3.1.3!
I dont want to brick my phone, so Im asking:
Is it possible to downgrade from 4.0 to 3.1.3? AND
Is it possible to activate after downgrading using a jailbreak+unlock or such?
I cant activate using a SIM cuz I have no access to the AT&T network, and am using a jail broken and unlocked activated 4.0 right now.
Thanks,

 

[WilliamG1]

Downgrade it to 3.1.3 which works quite well and is fast.