Radeon HD 7570 not performing like it should...

I have a Compaq Presario PC SR5710Y, its an old desktop from 2008. I upgraded the nvidia nforce 430 onboard graphics to an AMD RADEON HD 2400 it helped me with using windows alot smoother than what it was but then I decided I wanted to play some light classic games like cs go or halo custom edition which worked fine on the 2400 but were on the lowest settings chugging along at 30 fps keep in mind that the 2400 gpu has a tdp of 20w, I then decided to upgrade my gpu so that I could bump the settings up a little bit more so I bought a Radeon HD 7570 because it was benchmarked at similar areas to the gt 730 which I didnt want to buy for 60$, So i got the 7570 for only 20$. Its tdp is 60w after I installed it I had some difficulty finding the right driver since only dell has the driver for this gpu but supports only windows 7 and 8, I have windows vista 32, so I downloaded the drivers from amd's website and went to my gpu properties and it said VGA Graphics Adapter I said browse files for driver (this was after I installed it and nothing happened) and changed the directory and reinstalled it to my surprise it worked and It showed up as a radeon hd 5450 instead of a 7570, I didnt mind that because it was the correct driver and when I downloaded the driver for the 2400 it had a similar situation but it said 3400 and it still worked fine. OK, now that I got that out of the way I ran into one more hiccup my display settings didnt have an option to display at 1366*768, my monitors resolution, closest one was a 1024*768 which looks very pixelated and hurts my eyes (Yes I did go to properties and unchecked dont show all resolutions my monitor doesnt support that way other ones would show this did the trick for my 2400 but not my 7570) I decided to test some things out first my computer was quite noticeably slower than it was with the 2400, even slower to boot, and on top of that it is displaying an incorrect resolution. I opened up some games, some games that were practically unplayable before work normally now just a bit choppy but the games that worked on my 2400 and performed well doesnt perform as well on my 7570, for example a 3d browser game called begone stayed at a constant 40 fps before with everything at low almost all maps, good. The 7570 however jumps all over the place but I can still turn up the settings and it will still play, what the 2400 couldnt do, so I know it has potential to work well. So it basically is choppy jumps all over the place fps wise and just seems slower than my 2400 but shows potential...? My specs are an AMD ATHLON X2 4450e, 3gb RAM, Windows Vista, 250w psu, AMD RADEON HD 7570. Ok so I have some theories as to what my problem may be and I would like a second opinion so I can go about fixing this the right way, ok. Either my psu (oem, and an old used one at that) is not giving the gpu enough power or the resolution is messing things up, which surprisingly did mess up the 2400 as well just not as much and i found the right resolution. The solution to both of these are either buy a new power supply or buy a new monitor to fit the resolution supplied. I have some other Ideas too, it could be a bad driver or it could be I haven't correctly uninstalled the old drivers, could you help with supplying me with an application that could help? And lastly it was a used gpu off ebay, maybe there is something wrong with it phyisically, maybe something got smashed? Or the gpu literally is bad??? plz any help and opinions will be appreciated just plz dont say upgrade i know that already I just wanted to know what I can do to fix this.