My questions are, if I buy the PSU and buy the GPU, will I need to buy anything else? First time installing this on my own also. Will the GPU work on my motherboard? Will the card "fit"?
No, you should not need anything else.
Your link to the pc shows a standard M-ATX motherboard with a pcie-x16 slot. That is good.
The only thing that gives me pause is that the HD5450 shown appears to be a half height card.
Yet, the case looks like full width.
Measure the height of the slot in the rear opening where the graphics card will be installed. A standard slot will be about 10mm high.
While we are at it, measure the opening of the psu. A standard atx psu will be 150mm wide, 85mm high, and vary in depth from 140mm up to 180mm.
The seasonic I linked to will be 160mm deep.
with that cpu it would bottleneck the 660ti a lot and only be able to use part of the 660ti
I don't much like the term "bottleneck". It implies that bad things will happen.
When you upgrade anything, you get better frame rates. It is just a matter of diminishing returns for your dollar.
To help clarify your options, run these two tests:
a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 50%.
This will simulate what a lack of cpu power will do.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
set to 50% and see how you do.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
Tell me what you know about my CPU. I will run these tests and get back to you.
The link you gave said you had a phenom X4-945. It is a decent quad core cpu.
I have little doubt that a graphics card upgrade would do you a world of good.
What is not clear is how sensitive your games are to the need for cpu power, and for gpu power.
The tests I suggested are one way to get some insight into that equation.
If you find that you are truly cpu limited, then you are facing a cpu and motherboard change. Just upgrading your X4-945 to a faster X4 would not bring a substantial difference. You would be looking at a change to a Intel i5.
The pc is new, don't want to have to completely rebuild it. Maybe I should just buy an i7 and a new heatsink and fan for around the same price as the gpu and psu. What do you think, pretty sure my CPU will bottleneck.