Depends... as always.
Cards are limited by the PCI bus. Expect to get 2-3 times performance increase before tuning unless you move to 64-bit or PCI-Express.
If you tweak the frame size, etc you may be able to eek out up to 4 times but any more than that is pipedream land unless you are using integrated controllers or a faster bus.
Just don't go expecting 10x performance increase, but if you shift a lot of BIG files then you get to love the double or triple speed of gigabit over 100Mbit.
However that is all assuming that you can get them working properly. I have popped those cards in 2 linux boxes and 2 XP boxes. The expected 2-3 times performance increase between same OS, but down to 10Mbit performance (and errors in files transferred) between XP and Linux when pushing files to the Linux box. Very weird behaviour, made even stranger than the perfomace increasing 20 to 30 fold when you start copying a second file!
If you have a mixed XP and Linux environment I would avoid them until answers are forthcoming on the problem. And its not just me that is experiencing these problems: (read mrqx's post) http://www.linuxquestions.org/questions/history/69418
So - yes - it is worth it IF:
- You get it working
- You are transferring large files (100'd Megabytes) regularly