I can’t see the actual details of your PC when I click your first link. You will have to type or paste in the specifics, such as CPU, hard drives, RAM, monitor, operating system, etc.
If you are buying an Intel 2000 series processor, that would use dual channel DDR3 RAM. I assume you are buying 4 GB, which will probably be delivered as 2 sticks of 2 GB each. Your motherboard should have 4 slots, but you will have to confirm that. The normal upgrade from 4 is to 8, rather than 12. Very unlikely you would make use of 12.
An ATI Radeon 6950 is about $250 US in the US, so yes, Dell is a ripoff.
That is a very powerful graphics gaming card. By the time you save enough money to buy it, there will probably be a better graphics card available for the same money. So don't make a graphics card upgrade decision UNTIL the money is actually available.
If you bought that card, you would probably have to upgrade your power supply to run it. Some Dell PCs have oddball connections for the power supply. This is another reason to have a PC built locally using standard parts rather than oddball Dell parts. I don't know if that particular Dell uses anything oddball.
I don't know if that Dell case is a full ATX size. It may be a skinny case of some kind and that may mean that a regular ATX power supply would not fit. More reason to use a local builder.
That’s a good SSD. I assume you would get it later as an upgrade? If so, I would wait until I had the PC so I could confirm the hard drive mounting setup. It may be that you could use a 3.5 inch SSD rather than a 2.5 inch model.
Base: Intel® Core™ i3-2120 Processor (3.30GHz, 3MB) - Intel Core i7-2600 Processor (3.40Ghz 8MB)
Microsoft Operating System: English Genuine Windows® 7 Home Premium 64 Bit
Operating System Recovery Options: OS Windows® Recovery Media Not included
Enhanced Service Packs: 1Yr Collect & Returns Warranty – No upgrade selected
Online Backup: DataSafe Online Backup 2GB - 1 year licence
Microsoft Application Software: Microsoft® Office Starter: reduced-functionality Word & Excel w/ads.* No PowerPoint or Outlook
Protect your new PC: McAfee® Security Center* 30-Days trial version
Accidental Damage Support: No Accidental Damage Support
Memory: 3072MB Dual Channel DDR3 1333MHz [3x1024] Memory
Optical Devices: DVD +/- RW Drive (read/write CD & DVD) with Roxio and DVD Burn software
Hard Drive: 500GB (7200RPM) SATA Hard Drive
Video Card: Graphics : Integrated Intel® HD 2000
Sound Cards: Sound : Integrated 7.1 with THX® TruStudio
Monitor: Display Not Included
Mouse: Dell Optical Scroll USB (2 buttons scroll) DUAL TONE Mouse
Keyboard: Dell™ USB Entry Keyboard - UK/Irish (QWERTY)
Dell System Media Kit: XPS 8300 Resource DVD
Speakers: No Speakers (Speakers are required to hear audio from your system)
Shipping Documents: English - Documentation with UK/Ire Power cord
Standard Warranty: 1 year Collect & Return Hardware Support included with your PC
Call Dell Experts: One free Dell Expert call to help with your PC queries within 60 days of purchase
DataSafe: Datasafe Local 2.3 Basic
Most of this is not needed, but why not
I chose the GFX card because it was the only one I thought was compatible
Crucial says the max RAM is 16GB, which I will try figuring out how.
I'm getting the SSD after purchase yes.
I have no idea how to upgrade my power supply or anything
Edit; the RAM has 4 Slots and the Maximum is 16 GB so maybe 4 sets of 4GB Ram?