Short answer, I wouldn't recommend trying to upgrade the system as it is. As far as I can tell your case won't allow for the components you want. You'd be better off trying to rebuild in to a new mobo/case/psu with the parts you have now.
That said, here's were you stand if you can find the parts you need.
PSU: The 9020 SFF uses a 255w PSU which isn't going to be enough for what you're looking to do. Your model is a FP16X by the way. I'd look it up so you can physically see the shape before you start buying a new PSU. You'll need to find a 400w/500w + model depending on the video card. Also, the 9020's use a proprietary power connector (thank you Dell) so after finding a PSU you'll need one of these 24pin to 8pin adapters
http://www.moddiy.com/products/Dell-OptiPlex-9020-PSU-Main-Power-24%252dPin-to-8%252dPin-Adapter-Cable-%2830cm%29.html
GPU: You'll need a half height card and likely a short card at that. If you can find a 400w SFF PSU that fits your case, you should be able to pop a 750ti in to it like this one
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125680
RAM: You should have 4 slots for 1600mhz DDR3 with a max of 32GB. You'll want to avoid anything with tall heat spreaders but beyond that you should be able to go for whatever you like. Best practice would be to go for a matching set with your existing RAM. To get a matching stick, you'd have to find the model and search it out. Chances are it'll be stupidly expensive because Dell. I'd recommend just picking up a 16GB kit for about the same price as a single stick of non matching 8GB RAM
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125680
HDD: You can fit a normal 3.5 drive in there. Grab a 1Tb WD blue and you're good.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822236339&cm_re=1TB_WD_blue-_-22-236-339-_-Product
*Edit, if you can find a PSU try and make it a modular one. You are going to be seriously pressed for space to fit the cables with a SFF case.
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