Low Frames in Rfactor!!

hi everyone,
I play rfactor online and I experience low frames when there is more than 15 cars running on a track. I have an old p4 3Gb running at 3.3gb with a sonic tower is is very stable . I run a p4p800 deluxe mobo,with 2.5Gb gerneric ram v-data brand. x1950 pro 512mg .When i have less than 15 drivers in a server on a track i get around 30-40 fps which is just making the sim run ok. But when more people come into a server in the sim ,my frames start to go down litlle by little to around 25-30 fps and sometimes i have to quit the game session and wait and come back later when there is les people running the game. I know my pc is an old crappy p4 ,but some other people in the same game run old agp pc's like mine and they get 60-100 fsp with 40 cars in the game. Its very frustrating when you see this. their specs are:

athlon 2800 oc to 3000 barton processor with 2Gb ram ,9800pro 256mg ati with the nforce chipset on the mobo .

I have had a good talk with the guy who has this amd pc and ,he is amazed how crap my pc runs at.!!! I have even done a few benchtests and rated my scores with other tomshardware people with the same pc config as mine and my results alittle lower than their's in scores . also i have a 500w psu with a max 30A rail for the videocard, which is brand new and it replaced my old 450w which i had no difference in performance as well.

Would anyone have any ideas why this pc would be lagging alot in frames as i have found others with the same config as my pc to have alot better in performance when running rfactor.

any surgessions would be appreiated.