The 8600 video card was introduced at cart F.2. An E.2 cart would have shipped with 7600GT cards. However, when the 7600 became obsolete, Philips had to change to the 8600, and had to make changes to the UAVIO and the application software to support the new video cards. Which video card(s) you need depends on the UAVIO that is installed. Your system apparently has a UAVIO with part number 45356132529X.
AUE has both new and repaired 8600GT video cards in stock. My opinion is that it is OK to replace just one at a time, in general. If the site is remote or if uptime is very critical, then I might replace both. The cause of failure on the video cards is usually overheating of the video processor, and keeping the cooling fans on the video cards clean and regular cleaning of the main system air filter is critical to the reliability of them. For a quote and information on how to purchase, you can send an email to Sales@AUEtulsa.com or call 918-628-2831.