You can up grade to a GTX 560 Ti but you will need a 500w PSU to drive it I will give two links to those PSU's. Your power supply is no good and You need a min of 400w to power that bad card of a GTX 550 ti.
They cost to much money and they don't give you what you want. I know I have two of them in SLI. However it is better than your GTS 220. If Dell put a 460 in one like yours they put in another PSU to run it not yours.
You will be happy with the 560 ti at 1GB Vram asuming your resolution is 1920 x 1080. Good Luck on this.