Maximum PC

So I need a lot of help regarding video cards and power supplies and would super appreciate anyone who's able to help me with this.

This is my computer. I've had it since New Years, I love it, etc. The specs are listed but notably it has a Intel Core i3-2120 3.3GHz 64 bit Dual Core Processor, 4GB memory, 300W power supply, and as far as expansion goes, it has 1 x PCI, 1 x PCI-e 2.0 x16, and 2 x PCI-e x1. I had recently quit World of Warcraft before getting this computer to replace my fried laptop, and mostly I've been using it for entertainment and... onlinery. Writing, blogging, girly things, you know the stuff.

However it has a built in Intel HD Graphics card and I'm getting back into gaming. Sort of. Okay I'm playing the Sims 3 and here is a handy graphics card guide and you'd laugh but honestly the game is more demanding than you'd think, if you take a look at the requirements there. Aaaaalso with MoP out today who's to stop me from getting WoW again, amirite. Or Skyrim again. Or anything, really.

Problem is the 300W power supply. I ended up buying a NVIDIA GeForce 550 Ti, and you might notice it says nothing about required power specs. I ordered it, it got here a day later (I love you newegg), and now right there on the box it says min required 400W power supply.

So... crap. The tech friend I have says it should be okay with the i3 processor but honestly, I don't want to fry my computer. My question here is, should I backtrack and try to find a 450 Ti, even though it's now two generations back? Or should I get a larger power supply (currently looking at this, though I know less about power supply stuff than I do about vid cards), and either keep the 550 Ti or go with something a little more powerful, such as the 560 (or with two fans!)?

I just don't know what to do with this augh. Budget is negotiable - I wouldn't mind dropping money to get a better power supply nor would I mind buying the 560, even if I wish they were slightly cheaper. Probably will want to avoid current gen stuff if only because it's likely to be the most expensive and considering how little I actually game, I don't think I can justify it.