What texture mode are you running the game in? You can find it in the video settings on the front screen GUI. 128, 256, 512, 1024, 2046, 4096. What video card are you using? How much RAM does it have?
Offline I can crank everything up including adding some AA from my video cards control panel in 4096 Mode. In reality with my 1G card I'm in 1024 Mode to eliminate stuttering over towns, fields, and in furballs online in the alpha arenas. Adding AA would eat away some of my FPS.
You need to give more info or even attach a DXdiag.