PS. i did the same thing about a year ago. I dont know what my HP model (DC550)is off the top of my head but it has an old Pentium D Dual 3.6 with a 200watt psu. I put in a 9800gt low pro and 4x1gig sticks of 800mhz ddr2. The thing has been running like a champ for 2 years now. Normally doing netflix and such it only pulls 130watts or so from the wall using a Kill-a-watt. But when playing MW3 on medium at 1080 it pulls 205 watts. I know the PSU will blow sooner or later but in the mean time i having fun.