sorry but that is a horrible PSUhttp://www.hardwaresecrets.com/article/Cooler-Master-Elite-Power-460-W-Power-Supply-Review/1005/9
This unit can't deliver its labeled power - like Elite Power 400 W, it can only deliver up to 420 W. To make things worse, this unit is sold for USD 10 more than Elite Power 400 W - an extra 33% profit for Cooler Master for counterfeiting their own power supplies.
also the pentium D will bottleneck the 550ti.
with that said, you may try to see if the PSU will handle the load; it just may but if it doesn't you could lose more than the PSU. but really you ought to go for a 7750; lower power and about the same performance.