blznhot18
You will have no problems running the GTX 550 Ti with the stock 600 watt PSU from HP. Although it won't really hurt anything if you know how to change it out, you really don't need to switch PSUs. HP uses Delta for those units, and they are not too bad at all. I wouldn't take one over a Corsair or Antec PSU, but they are decent and can easily handle anything you want to add to that rig, like a TV Tuner card or more RAM. 600 watts is plenty. It's just as good as the CoolerMaster you have (which are only so-so).
I too have a brand new HP Phoenix rig with the i7 2600. I immediately swapped out the GTX 550 Ti card for a overclocked XFX Radeon HD 6950 card that I already had. I have been running it for three weeks now without a single glitch or hiccup. And I have 16GB of RAM, a 2TB HDD, Blu-ray player and the liquid cooling (I did not get the SSD, but I will be adding one later myself).
Again, you don't need to replace that PSU, but since you asked and you sound like you want to- here's my recommendation for a good quality PSU for the Phoenix- its a fantastic unit from Corsair:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139020
And you cannot overclock your rig. Although yes, it certainly possible (but quite difficult), HP did not give you any options in this BIOS to change the multiplier, plus the i7 2600 is not a unlocked part. That would be his cousin, the i7 2600k of course.
By the way, I hope you are enjoying your computer. My Phoenix is awesome, and superfast!!! I got a great buy for about a $1000 through my work, and I haven't regretted it at all. I almost bought a similarly configured Alienware which was a lot more $. Glad I picked the HP. This things runs cool and is quiet as a mouse. I'm getting 70+ FPS on BF3 with everything on Ultra and 4xAA, and the Ambient Occlusion setting turned on. It's a really decent gaming rig for a prebuilt.