So i'm really bent on whether for my birthday i should get this computer (Gateway SX2855-UB12P Desktop Computer) at the micro center by me its around 250$ and at Walmart 190$, idk whether i should get at Walmart will it matter? I like the really low price as my parents aren't too happy about me wanting the 400 dollar computer i thought of previously. I know obviously the computer alone wont play anything whatsoever. It was meant as a starter with potential to later be upgraded. So i was thinking this computer a new power supply (probably 700 or 600 watts) an i-7 processor, 20 inch screen, a Sapphire Radeon 7870 heard has great cooling, and either 8GB or 16GB of ram (says 8 GB maximum, but i thought i saw individual 8GB 1333mhz ram) Any other suggestions would be greatly appreciated such as maybe cooling.
The problem with purchasing off-the-shelf systems for later upgrades is that OEMs often use crippled motherboards to save money. With some boards, you cannot upgrade the processor. Some boards do not even have a PCI-Ex16 slot. In your case, the system already states a maximum limit of 8GB of RAM. This is a motherboard limitation and not a RAM module limitation.
Since this appears to be a small form factor PC, I doubt you'd be able to upgrade the power supply. You certainly would be limited to a low-profile PCI-E graphics card (assuming it actually has a PCI-E card slot). This is not a system you purchase for upgrades.
The Dell Inspiron 660 suffers the same issues that your previous system has in that it's a small form factor PC (barely large enough for a DVD-ROM drive). You need to be looking at systems that come in at least a MicroATX Mini-Tower case.