Upgrading my Asus Essentio

Hello, I have an Asus Essentio I bought back in February, and want to upgrade it's gaming performance. I can't give an exact link, as the model I purchased isn't on the site anymore, but a very similar one is this

http://www.bestbuy.com/site/Asus+-+Essentio+Desktop+-+4...

I have 750 GB hard drive, an AMD Athlon II X2 220 Processor, but other then that, I think they are the same. I know it's a lower end PC, but I don't have alot of money to be getting a high end one. When I bought it, I was asking the guy at Bestbuy about upgrades, and he was saying I would need to upgrade the Graphics card, power supply, and processor. The game that this PC was and is revolved around, is SWTOR. Right now, I can run it OK on minimum settings, but it lags alot when there is to much going on at once, and is pretty frozen with smoke, moving water, and explosions nearby takes place. It would be nice to get no lag when playing, and maybe even be able to play on higher settings. Just no lag is the biggest want. I really do not know alot about hardware, the sales clerk suggested a certain graphics card and power supply, but can't remember what they were, but the graphics card was around $150, and the power supply, was around $100. I'm willing to go higher on those, just as long as my computer can use them. I was thinking of an Intel i3, 3.2 ghz, i'd even be willing to spend more on that and upgrade to an i5 if my computer can run it. So my two questions are,

Which graphics card, and power supply (if I need the power supply) should I get. The most I would be willing to spend is $300, if it's necessary.

Which Processor should I get? I know for sure I want Intel. i5 or i3? Specific model?

Thanks in advance for the help.