So how do i check if a certain graphics card is compatable and will fit in my computer?

Ok i thank everyone for all your help. I have one more question. I just opened up my computer and it is unbelievably dusty. So how much do you think this could be causing a lot of my lag?

Yeah my FPS when not in combat is around 40-60 flashing green and red. but as soon as combat starts thats when i have problems and its just red. Any suggestions on a video card with the similar cost/preformance as radeon 6850 but you think would be better for swtor? I also have 8g of ram so i think i am good their

allright thanks a lot. radeon hd 6850 good? wont bottleneck too much?

My cpu is an Intel Core i7 860 2.80GHZ Graphics card is ATI Radeon HD 5700 series I know nothing about computers really but the game extremely laggy in combat. Any help is greatly appreciated.