Nvidia users can still run using DX 11 mode; however it will cut down to DX10 (without all the fancy features of DX11). I switched from DX9 to DX11 (which downgraded to DX10) and I noticed a considerable difference in graphics.
1: Play DX9 unless you have a combo of Vista/Win7 and a Radeon HD5k series video card. DX11 is only available on Vista and Windows 7 (it is NOT exclusive to Win7).
2: Update drivers as applicable (Video, audio, etc.)
3: Disable background processes that are not needed. This includes anything that can eat up CPU processes such as automatic updates for Antivirus software, and anything else that can interrupt the game.
4: Ensure your rig meets or exceeds the RECOMMENDED requirements (Minimum WILL give issues, as it always does on games).
Plus if you are Launch the Aliens Versus Predator first option that is DirX 10. reason be they have 3 choices. cause a lot of video cards support DirX 10 and 9. others can't so you most make sure you select the one that works with your video card.
I am running a GTX 280 never had a problem with memory leak nor slowness at all.. it works great for me...