if you are going the prebuilt route at least go for a website/shop that puts together quality components for you. Do not make the mistake many people make and try to get a deal on ebay or something like that, you will just end up being disappointed.
Another thing you can do is build it yourself. I know it sounds daunting to someone who has never considered it, but there is not much more to it than connecting 8-12 parts, that can only be connected in a very specific way. Watch some of the youtube videos (there are a few good ones by newegg I believe) and pretty soon you will have a good idea of what you need to do.
The benefit is not just that it saves you some money, but also that you will know enough about your PC to maintenance and upgrade it (since you already expressed and interest in doing this in the OP).
And yes, you can upgrade pretty much any component based on what you need at that time. For example, in my current PC I have changed the GPU, motherboard, processor and RAM over the years (while keeping the case, PSU, HDD etc.) The most common upgrade that has a direct effect on how well your PC is able to run games is the GPU. It only requires you to connect one or two cables and slide the GPU in the dedicated slot on the motherboard.
I5 3570 | GTX 760 | FILCO Majestouch 2 tenkeyless | Zowie FK | Asus Xonar DGX | Kingston 120 GB SSD | Sennheiser HD 518 | Samsung S24A350H
Building is pretty easy, especially if you already have some stuff like a monitor and keyboard. If you have a computer tower laying around you might be able to just empty it out and reuse the case and power supply.
most everything can be upgraded but a CPU upgrade will require a new mother board.
the hardest part of building a pc is what parts to get and this board will help you by giving you a list of parts to get just give us a budget and an idea of how good you want it to be (this way we can say if your budget is either way to high or way too low).
Xbox GT: Zarosian 3DS ID: 1848-2118-1570 Origin and SteamID: Zarosio
If your technical knowledge is average to high, you may consider building one yourself.
Extensive research is the key to building a system that is stable and fast.
Go to website of PC retailers and mark down the part number of components (especially for mobo, CPU and video card) of their pre build systems that resemble what you want. Then google those part number for review.
Also read the reviews on newegg and component manufacturer forums. Google part numbers of two components together. You might find people post about compatibility issues.
YouTube also has lots of review and plenty of tutorials teaching how to build a PC.
Forget about XBOX One. This is my XBOX720: i7 2600 @3.4Ghz | ATI 6850 1GB | 8GB DDR3 | Asus P8P67 LE Now playing: Yakuza 5, Bioshock Infinite