The 6600GT is your weakest link in that system. Earlier this year, I had given my better half my Xmas 2005 system (Opteron dual core 165; 7800GT; 2GB DDR), so I was using my summer 2003 system (Barton 2800+; 6600GT; 1GB DDR) while sorting out my new quad core setup...
To run Rfactor at a decent framerate, I was having to run a medium/high mix of settings, with a tendency towards lower settings on the player/opponent detail. I also had to reduce the number of visible opponents to less than 10. Some mods were unplayable, such as the rich car detail of the PCC2007 and GTO 288 mods, even when reducing settings to minimum. Due to my aim of keeping the framerate above 50, I ended up running in DX7 mode.
A graphics card upgrade would revive your system for rfactor 1, a replacement for £100+ (8800GT or newer in Nvidia range; 3850 (?) or newer in ATI offerings) would almost certainly allow maximum graphics detail at good framerates. This is presuming that your motherboard uses a PCI-E graphics slot, the sucessor to AGP. If correct, you could always transplant this new PCI-E grahics card to a newer unit, when you felt the need to upgrade.