8GB DDR3-1066MHz SDRAM [4 DIMMs]
640GB 7200 rpm SATA 3Gb/s hard drive
Integrated graphics - Intel(R) Graphics Media Accelerator X4500
I am thinking of getting a graphics card, something cheap actually. I only play Pro Evolution soccer 2009 from time to time, so I guess 128 MB would be enough. Any suggestions? And would I need to change anything else, like the power supply? (I do not want to, since I do not have any money to waste these days. So keep that in mind )
Well, I don't believe there is anything modern on the market with less than 512mb of Vram, so that is something you don't have to worry about.
Secondly, without knowing what your PSU actually is, it is hard to recommend a video card that will work with it, but assuming it is the PSU that came with the HP prebuilt, there probably isn't a lot of headroom. You will be limited to some low power draw, low cost cards, anyhow, which is fine considering a limited budget, anyhow.
I would suggest a 4670 DDR3 or if you have bit more cash to spend, a 5670.
I hadn't checked the prices on these products for awhile, and it seems that the 4670 has gone up in price (could be found for $50 after MIR and deals last fall/early this year) and the 5670 has dropped already. For the $9 difference I would probably take the 5670.
The wattage recommendations are to compensate for poor quality unbranded overrated PSUs. HP generally has higher quality PSUs than the lowest common denominator, and should be sufficient to keep your system powered.
If you insist on replacing the PSU as well, you are correct, just make sure everything that was plugged in before, is plugged in with the new one. It is more complex than say, swapping out your videocard, but its just a matter of repetition.
HP PSUs have come standard with overpower protection for quite some time. It is the overpowering that can occur during a PSU failure that does damage to the rest of the system. 99 times our of 100, when an overpower protected PSU is comprimised, it will restart the PC under load, or just not turn on.