What was the question? If its "is your computer compatible?", then on paper yes, however you didnt list your motherboard - you need a free PCIE x16 slot. The 550Ti needs 1x pcie 6pin power connector, so make sure you psu has one.
Xion is not a brand im familiar with, however a quick look around the web shows that its probably ok for your system and a 550Ti.
The OP stated that they wanting to upgrade from a GT240 which being a PCIe card would suggest that their motherboard has the requisite slot.