Upgrading Acer Aspire M1100 for Gaming and Windows 7

Yikes, yes sir, you'd need a new computer. Yours. The case might be worth something though...

Don't know that you can get anything that will play BF3 with 400US$ o.0

Could try to build it yourself

well, you'd need a fair CPU (doesn't have to be anything amazing, usually), a high end Graphics card, and a lot more Watts on the PSU. The hard drive might be fine, mostly only effects loading times. So you might keep the hard drive, but may need to replace everything else. You could keep the motherboard and just build off it with older high-end compatible parts. That can be very hard, hard to find, hard to do. Compatibility might still be an issue.

To build your own computer can make parts much cheaper. I really recommend for simplicity and value, building your own. You will need to do some research before you decide what you want to do, and how you want to start. You might wait till you have more money to spend, especially for those new high-end games.

While not completely sure, I would guess your case can be used. However, motherboards, graphics cards, PSUs, etc al differ in size. There are many different computer case sizes, from custom to small to medium to large and beyond. You are just going to have to know the size of what you buy for it and whether or not it will fit. I just buy full tower sized cases in order to ensure that my oversized graphics cards fit, that wires aren't bunched, and that air flow runs smoothly.

You might check your case and see if you are able to remove the components easily. If not, then it will be difficult to put componants in.