Let me try to clear a few things up: As the mod said a lot of motherboards are ATX form cases.
With that said, having an ATX form case does not enable you to upgrade things in your PC.
What you really want to know is what your motherboard can handle. How many slots of ram does it have, what type of RAM can it take, how fast is the ram that it can take, what kind of processors does it accept, does it have an AGP slot, is it 2x 4x or 8x? Many questions you have to answer in order to know about upgrading.
I will say however that it is easier than you'd expect with a little research you can upgrade your PC.
On the other hand, upgrading is not always the best solution. To explain, sometimes you will get more than you bargained for. A long time ago I had a compaq presario. I wanted to upgrade my Video Card, and I found out I had no AGP slot on my motherboard. What I did next was discover that they made PCI video cards for motherboards with no AGP slot like mine. However, it was not a good buy because the pipeline of a PCI slot as opposed to an AGP slot was huge. I ended up spending just as much money for an AGP card, and I got about 1/10th the quality and performance as I would have if I bought an AGP card (if my motherboard would have actually had it). And then even after that I found out I could not support the video card without upgrading the shoddy power supply that compaq provided in their presario. I think it was 65 watts (very sad) and I had to buy a new one of those too.
So, it depends what you want to upgrade and even then you may be better off buying a new pc (I would suggest going with something that you custom build: if you don't know about the different parts it's always good to do a little research and learn), or your BEST bet, is buying the parts and making your own. I would have no problem walking you through how to build one. They are quite simple: like a big lego set for adults.
At any rate, hope this explained something.