SH3 computer specs

Hi im new to SH3 just buying it the other day. My question is that i have an older computer and SH3 runs choppy. I have turned off high detail and lowerd particle density but the game still runs kinda of choppy when i switch stations etc. I dont mind the game not loking top notch, I just want it to run smoothly. Also I had friend install it and he installed the gray wolves expansion pack. I dont know if this make a differance or not

thanks

update: my spec are Intel Celeron D 3.33ghz cpu, 512mb RAM, ATI radeon xpress 1100 series