Adding a hard disk and RAM, i've used OWC but wondering if it makes sense to add an internal 250 gig (Hitachi, Maxtor, Seagate) or another firewire but the xfer rate is a bit better internal, no?
Internal drive is better way to go. There's only 1 firewire port on your mac (there's more conenctors but internally 1 port) so you may not get the speeds you think you will. Technically Firewire drives aren't offically supported for FCP.
I just did the same thing on a G5 and internal is the way to go. I also have an external OWC drive that works great for files and photos and such, but an internal ATA drive is best for DV. I have a newer G5 and its such an easy install. They even include the right screws in the Mac chassis. I think it took an entire 3 minutes to install. I got the 250gig Maxtor, and I love the set up.
In my G5, I have a 160 gig system drive, a 120gig Maxtor drive that holds my Video Projects that I am currently working on, and a 120 gig External Western digital Firewire drive that holds my completed projects for storage. The G5 has more than one FW port. I have two 400's and one 800 port.
Yes, you have multiple firewire ports but internally they are connected to the same data path. This means if you plug in a slow device to the firewire 400 you'll be limited in speed. This is especially true if you input your camera into a firewire port and then go out another port to go to a firewire drive.
This usually works with fast machines but you may find some problems when capturing. If your machine is focused on editing go the internal hard drive route- cheaper, faster and more reliable. If you need to go external (such as on a PowerBook) you might consider getting a firewire card.