Combine two ethernets?

[Xan]

Hi,

I have two network interfaces: eth0 and eth1.
I just want to know if it's possible to combine two interfaces for 
downloading (and uploading) at the same time in the two interfaces. That 
is, can ubuntu use the two interfaces at the same time?

For example I think:
a) ubuntu automatically uses the less used interface. If it's possible, 
we could have two balanced interfaces (equal traffic bandwitth)
b) we could specify which program uses what interfaces. For example, use 
bittorrent in eth0 and web browsers in eth1....

And the question is: if it's possible, is it _easy_ (for desktop users)?

Thanks in advance,
Xan.