Albeit that video card is a bit on the lower end...if people with 7300GT AGP graphics cards can run the game decently there is no reason that 8500GT can't.
No offense to Ripley or Charge...tossing money at something is not always a viable solution for the average person...$90 is not a paltry sum to throw into a household utility item, especially for someone with a family to feed.
Pewter, you have 2 obvious things going on...as was pointed out, your paging file usage is huge...just idling (no open programs) that system shouldn't be running anymore than somewhere around 500 to 600 MB paging file usage. You need to get rid of all the "bloatware" running on Vista (sidebar, widgets, aero interface, etc...)...set windows peformance settings to run at max performance, not best graphics...if your anti-virus is a Symantec/Norton, McAffee or Trend Micro product, uninstall it and get a better "freeware" anti-virus...get a good registry cleaner and clean out your registry...then, before going into the game, turn off your anti-virus and use the game booster program.
Turn your video card resolution down, your dxdiag says you're running it 1920x1080 @60Hz...that's maxed out...1440x900 is more than adequate, and you might be able to set your refresh rate to 72Hz (you will start with 72 frames in game)...set your game resolution to 1280x920 with 512 textures...turn off all shadowing and detailed water.
Your frame rate should stabilize...if not, use a lower in game resolution (1152x860 ???).
