PC Parts upgrading, Help!

[Joshl]

Hi, I recently brought a pre built pc online made for 'gaming purposes'. And while it does run games fairly well (my main game is World of Warcraft) I have to run on low-medium graphics and getting about 20fps average.
Because of this i decided to upgrade, I checked with Windows Experience Index and my results were as followed;
Processor : 7.1
Memory (RAM): 7.1
Graphics 3.3
Gaming Graphics. 5.0
Primary hard disk: 5.9,
Because of these results I decided to upgrade my graphics card to Asus Nvidia GeForce GTX 750 Ti Graphics Card (2GB, GDDR5, PCI-Express 3.0)
This arrived today and I found it doesn't fit in my PC case (yes I know, I should have measured!) I believe my PC case is a slimline (Optiplex 780 is what is written on it.) It also seems like it it clashes with the black box on front of pc (connected to the fan bit under the power button.) My power supply does not seem to have any spare cables either.

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So basically - What I am asking is ;
What power supply should I buy?
What case should I buy (to fit all my current parts in + this new graphics card.)
and Do I need a new mother board because of this black box thing (sorry, completely clueless!) being slightly in the way of graphics card?
Thanks for taking your time to read this. And I would be extremely grateful with a prompt response as that means I can buy the parts asap! I hope this is clear enough for you to understand.
Here are my current parts ;
System manufacturer - Dell inc.
System Model - Optiplex 780
System type - x64 based Pc
Processor - Intel r Xeon r CPU X3220 at 2.40GHz, 2394 MHz 4 core(s)
Not sure if that is relevant or not, but thanks again
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[Threshold12]

First, that black plastic is a shroud that is covering up the CPU heat sink, and is there to block air passages so it only goes one way out of the case. You can remove if you switch to a bigger case.

Second, from what I can tell unfortunately the mobo that is used by the system you have, uses proprietary form fact by dell, so you can't just stick into a new pc case. Buy some mobo stand screws and a new PC case. You'll have to drill new holes in your case to match up with the ones on your mobo.

Lastly the power unit. I am not to sure how many pins your mobo requires. If it is 24, then you can buy a new power supply. If it is 20, then you'll need an adapter or a power supply that can split mobo cable from 24 to 20 + 4 pin.

[Joshl]

Thanks for the reply. I have little clue about computers so I guess I'll just stick with what i have got rather than trying to build a whole new case. But thanks for the informative reply. If you was in my situation what would you do? How much is a motherboard that's at least matching my current components?

[Threshold12]

If you want to change the motherboard, you'll have to find one that supports socket LGA775. I am not sure if you are using DDR2 or DDR3 ram, you'll have to check for that as well.

If you have ddr3, amazon.com,
/Gigabyte-G41MT-S2PT-Micro-Intel-Motherboard/dp/B005OJRGYA/ref=sr_1_1?ie=UTF8&qid=1440967097&sr=8-1&keywords=LGA+775.

If ddr2, amazon.com,
/Intel-D945GCZL-Classic-Processors-MicroBTX/dp/B0009G2FX0/ref=sr_1_6?ie=UTF8&qid=1440967205&sr=8-6&keywords=LGA+775.

These are just some examples, as it is rather old socket and hard to find new parts. You'll probably have to use something other then amazon as it has rather limited selection.

If you have the money its probably best to get new parts and get new mobo, cpu, and ram, but if you want to reuse parts youll have to search around.