I need some serious advice.... video card help!?

so here is the situation. If any of you have some of my previous questions, then you know my problem! If you dont heres the deal! on my computer the two main games I play is Rollercoaster Tycoon 3 Platinum and The Sims 2 with all expansions and Stuff Packs! It works fine but one day this wierd staticy type stuff appeared on my screen! only on my games though! ( and google earth) not desktop and internet though! this staticy ness is wierd! it flashes when i move so it like gives me headaches! i can see through stuff in the static! for example in the sims 2 ill look at a wall and with the static i can make out the shapes of the bushes behind it or whatever is behind it! so thats kinda what the static is! i call it static but im not sure what it is! i guess its kinda like "snow" on a tv! so anyway! my brother who is the computer guy in my family said its probably my video card, which i kinda figured! BTW i have a Nvidia GeForce 7300 GT installed on it right now! so ive been looking into new ones, so the other day Best Buy finally had an AGP video card! I couldnt find one anywhere! there was one left! so i just bought it and figured if it wasnt what i wanted i could just return it! so my brother was in Cancun at the time so for the next 5 or so days it woudnt be touched! BUT... two days after I bought the card! the problem whent away! (figures right!) so i played for the next 3 days and it looked perfect the static never came back! my brother came home yesterday! and i explained to him the situation, he said if the problem doesnt come back to just return the card! BTW the new card is an ATI radeon HD 2400 Pro! not sure if its a downgrade or not, i dont really care as long as it fixes my problem! so anyway yesterday i was playing and the problem came back and it stayed that way for about an hour! then whent away and this morning came back again! and hasnt lefT! my brother says that hes not sure if its the video card any more because it comes and gos! so now I need your advice on weather or not the card will actually help or not and weather or not i should install the new card! please please help me! and sorry for the novel! i just needed to explain in full detail!