Here is a lot 3 NICs for less that the price 2 NICs fm Newegg. You will have a spare just in case.
http://cgi.ebay.com/LOT-3-Intel-Pro-1000-GT-Desktop-Adapter-PWLA8391GT-/380342300447?pt=LH_DefaultDomain_0&hash=item588e2aef1f
Have you updated the BIOS on the PC? Increasing the RAM on the system? Check the HP support for the cpu your system will support based on the system's BIOS revision, and that could assist you in buying a more powerful cpu, if possible. This could help in the systems throughput.
I have a HP DC5750 that came with AMD64 2.0Ghz, I updated the BIOS to most current, checked HP support, and with the current BIOS the system can support 3.0Hgz, I upgraded the RAM and CPU, added 2nd NIC intel pro/1000 XT PCI-X (the pci slot supports it) and never looked back.
as for the losing the internet connectivity, I believe the BIOS revision might fix that. I never experienced that before with pfsense in the last 3yrs of using it, both 32bit and 64bit in any version. I just made sure my hardware was compatible, BIOS was most current, and turned off the items in the BIOS that was not needed.
Pfsense Docs is a great place to start - http://doc.pfsense.org/index.php/Main_Page
Buy the book - great book.
Lastly search the forums, always my ticket to any issue I ever had.
Hope this help.