I had dell opti plex 755 2.6GHZ core 2 duo ... i wanted to buy a 3d card but for a good card u need a good power supply too .. if ur having tower casing then u should go for a good card like 9800GT which currently im having on my core i5 which i bought later... but if u have desktop casing so i m afraid u need to buy a normal card or change the specifications of your computer ... i dont think so ur pc will support GTX 550ti as the pci slots may be different ... the 9800GT has 2.0 pci express .. i dont know whether 550ti has 2.0 or 3.0 ...

Note :

if u want to run gtx 550ti then u also need a 550+ watt power supply else ur card wont even start ... even with the 9800GT im having 500 watt power supply ...