bignastyid :
The 7750 does not require a 6-pin power connection it draws all its power from the motherboard. CoolMax power supplies are garbage no matter what they say it's 80 plus rating is. The 7750 is a much better card than a gt640. The 7750 is faster and uses less power. As I said earlier the 7750 uses LESS power than the GT620 that you are currently running so it will most likely work. If you want to buy a PSU too this 300w Seasonic(http://www.newegg.com/Product/Product.aspx?Item=N82E16817151085) will easily run a 7750. AMD recommends a psu that can deliver 16a on the 12v rail(s), that Seasonic can deliver a total of 24a(288w) and Seasonic is one of the best psu manufactures around while Coolmax is one of the worst.
I have two 5770's here and both take a 6 pin connector. I doubt very seriously that the 7750 is drawing all of it's power off of the PCIe bus. As far as PSU's go, you best have some headroom on the rail(s). If not you are going have trouble with some games aor any other thing with intensive graphics. Sometimes that means more wattage on the PSU than you really need ( overkill ). At the moment, I have some serious overkill, but I can add a second GTX 680 in sli with no problem, as I have enough plus still have some headroom. As for the PSU, I have a list around here somewhere with who makes what. You would be surprised at what is listed in the first and second tier. For Instance, several of the Corsair models are actually made by Seasonic. I believe that list may be posted here someplace. The other thing is you linked to Newegg.com. The price was within his budget, but he is looking at Newegg.ca and it would have further over his budget than the one I linked to, in Canadian Dollars.