Honestly, a pre built PC is a terrible start for a gaming PC. The best recommendation would be a new case, new motherboard (most pre builts come with cheap ones). Salvage the RAM (but replace it in the future for a higher clocked high performance one), HDDs and buy the rest. Forget about upgrading the older machine.

If it isn't in your budget, I recommend to delay the parts and wait to be able to start a new build from scratch. You are saying that a laptop is out of your budget, so a gaming PC is probably WAY out of your budget. What GPU are you thinking about?

I'm a student, and I prefer a desktop/tablet combo most days (some days I'll bring my laptop in to download large files off the fast internet.) We mostly use the school's desktops anyway because they have the necessary programs we need for school work (MatLab, Mathematica, Programming environments, etc, etc.) For a laptop that will be able to play most games at even 900p you need to spend at least $800. For a desktop that can play all games at 1080p you need to spend $550-600 or most games at 1080p ($450-500.) I see more people with tablets in my classes than laptops these days. My laptop personally just asks as a media device these days. I use it to play digital blu-rays. 

He is saying a laptop that can play games is out of his budget.  Obviously there are decent $300 office laptops. 

I think he would make our life easier by saying what's his budget. I can't recommend and upgrade/build without a budget.

It seems to me as if he already went ahead and purchased said PC, GPU, Power Supply, etc so it is pointless now. 

So, what's the point of the thread? Making him regret the purchases? Or help him to actually build it? Now I'm confuse.

He's asking whether or not his processor - a xeon - will work with his motherboard and also asking if there is anything he's forgetting when upgrading this PC. 

Faster RAM only increases the bandwith, and without an iGPU this effect is basically lost. So you don't need faster RAM.

In any case, trying to upgrade an OEM PC is a pain in the backside, which is why they are generally avoided like the devil by PC gamers. Their Mainboards often lack features (or locks them away), their PSU tend to be shitty at best, cooling is low-price, loud and ineffective, and often the cases themseves are all of the above. To top it off they also often come with a metric crapton of useless additions such as Software you'll never ever need in your entire life and outdated connecters added for those offices which tend to upgrade their PCs once every century or so (Case of point: My mom bought a PC 3 years ago and it still had an LPT1, COM, and PS/2 connectors, which are not supported by the preinstalled Windows 8 anymore, but lacked any support for USB 3.0 or SATA3). Finally, they are often overpriced for what they are: Gloryfied office PCs with an added Graphics card.

I understand what your saying but I have a chromebook which is enough for writing papers and using the printer whn I need it. I actaully need a decent desktop for editing and stuff so buying a cheep gaming notebook would kill me and the performance isn't always great for general computing.