Upgraded this rig and isn't running the as good as it should

It was heavily outdated. Now I didn't upgrade it into super machine. I just wanted to be able to play some newer games. However the games I'm trying to play and running at lower FPS and choppier than they should be according to videos I've watched online of people running the same games on the same graphics card. I was watching videos of the basicly this same rig running for instance Batman: Arkham Asylum at full settings no problem. Yet my rig can barely do it at medium settings. I also saw it running GTA4 which will barely run for me.

Anyways here's my set up...
CPU: Intel Pentium 3.40ghz duo core
Graphics Card: Nvidia 520gt 1024mb fermi
Operating System: Widows XP Professional 32bit
Running 2 gigs of DDR2 ram

I'm not the most tech savvy so if you need more info I will do my best to answer you.
Thanks!

For a lot of the games you need a triple or quad core CPU in order to not bottleneck the components. Your GPU is also extremely low depending on your resolution and game quality.

There are alot of issues with yout build, 1 your ram is only ddr2 and you didnt even state weather it was 800mhz or 400mhz which can make big differance, 2 that graphics card is super weak unless you want to play at 1024x768,3 your processor is really old and is probably bottlenecking your graphics card aswell

what id suggest is to get an I5 2500k for only 200 bucks and then any 60 dollar mobo and 8 gigs of 1600mhz ram for 40 bucks, then youll know you wont be bottlenecking your gpu, then if you can atleast get a gtx 460 becuase i know it can run arkham city no problem in direct x 11, and please install win 7 64 becuase there is no point in having a direct x 11 graphics card and only using direct x 9 which is all xp can do