And again no. There is no USB to firewire that I've ever found to work. There are folk ready to take your money and then walk away.

As I read the service manual at
ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_xps_desktop/xps-8300_service%20manual_en-us.pdf

Page 38 shows 4 PCI express slots.

Are you saying you already filled all of those?
Bob