Looking to upgrade my dell optiplex 780 DT

So I have been doing a lot of research on the dell optiplex 780 just so I don't make any purchases of anything that wouldn't work in my PC. There are four things I am really focused on getting upgraded right away before I do any thing else like the hard drive and that is the CPU, GPU, RAM, and power supply. The CPU I am planning to get is the Intel core2quad q9650. Knowing that this is a power hungry card and seeing how I am going to overclock it, the card will make some heat so I am planning on getting an aftermarket cooler. Seeing how I can't really just slap on a coolermaster and be good to go I want to get this https://www.redplanettrading.com/Dell-RW134-PU469-R6852-C6119-Heatsink-and-Shroud-Optiplex-Desktop.html I think this would be adequate for overclocking and keeping it cool, but that's where I need the forums to make sure.  The GPU I will be buying a riser card made specifically by dell for the motherboard so that way my dell optiplex 780 DT will support full height cards and I will be getting a GTX 750 ti. The RAM I wont have an issue with just as long as I can find four 2GB sticks of Non-ECC DDR3 RAM running at 1033Mhz and my computer will support the RAM because I have a 64-bit OS and a CPU that will support 64 bit as well. This is where the problem comes in, I don't know what power supply to get. I want to get at least a 400 watt power supply because, even though it's a little overkill with the amount of power, it will be nice to know I wont have any issues in the future if I decide to upgrade again. So what do you guys think? Do you think I should change something? Keep it the same?