Buy a new computer, it'll be easier by the time you're done. The only things you'll likely be able to salvage out of your current computer are the HDD and ODD, which are a pittance in relation to all the other stuff you'd have to buy.
Don't even bother thinking about upgrading, focus on buying or building an entirely new computer.
And in the future, if you want to do PC gaming, building your own is really the only way to go. If you take your time, do your homework, and select components carefully, you should be able to upgrade the thing one or two pieces at a time perpetually. Eventually, if you save the parts, you can rebuild the original computer, and then have it as a spare, give it to someone, sell it, whatever.
So I bought this computer: Acer Ax1420g-u5832
I want to upgrade it so that I can play games like BF3, Skyrim, and other modern games with high settings and good framerate. To do that I will need a new case, gpu, psu, and possibly motherboard.
These are the things I've been looking at: Antec Three Hundred case, Thermaltake V3 Case, Corsair 600w Power Supply
I've looked at a few motherboards but I noticed that the ports on the motherboard don't add up to the slots for the ports on the cases. I was wondering if you guys could give me any advice on the best way to upgrade this computer. Thanks.