@carlos24 it's not worth updating your rig with a Xeon X5470 because you already have a similar cpu. It would be different if you had a Core Duo, or similar.

I also don't think you'll get much performance by upgrading the ram. And regarding the GPU the 750ti is better than the 550ti, it's strange that the second one performs better and it may be a driver issue.

I imagine you won't have much budget to improve your rig, but for sure you can find a second-hand Dell PC with an i7/i5 from 4th or 5th gen for little money and you can get a bit more out of that 750ti.

Regards!!