LAG, what caused it? lack of ram? slow cpu or vga? or what? mine is p4 2,66 1 gb pc 3200 visipro, x1600 256?

first it was only 512 mb, radeon 9550. it was too slow to play recent games, so i bought another 512 mb of ram and a radeon x 1600 agp 256 mb. doesn't lag while playing doom3 or quake 4 but slows down in certain area in the games (fps slow down). but it lags sometimes in need 4 speed mostwanted. i use medium config in all of those games. thanx 4 u guys out there!