you should be fine.
I'm only using a 250 Watt and I have maxed out the RAM, and it has a 500GB HD and two DVD burners.
my old PC was only 250 watts as well and it had three Hard drives, a DVD burner, and the RAM was maxed out.
however the old PC probably didn't have enough power as my third HD would randomly dissapear from the system.
but 400Watts should more then cover your setup if you add an extra drive- depending of course on your graphics card power requirements (IE: what graphics card do you have)
there are only a few reasons to go with a 500GB over a 1TB, aside from price, of course,one would be that if it fails you lose less data, the other BIOS compatibility issues.
of course if you have a proper backup failure is merely inconvenient rather then a stab in the face, so it would be kind of silly to use a 500GB simply because you lose less data.
And as far as BIOS compatibility any BIOS from a P4 onwards, at least, likely can detect and use a 1TB drive.
Additionally- if you install a 1TB drive, you can easily partition it to replace the two IDE drives letters with 80GB partitions- and then copy your data over, meaning you could remove the IDE drives. (why do you need them installed- do they contain OS's for a multi-boot setup?). This is what I did to replace my old three-disk 2GB 8GB 160GB configuration with my single 500GB originally, I just partitioned the new drive and copied data and then rebooted with it and changed the drive letters back as appropriate.
