right now id seriously recommend just getting a new card like a gtx660ti or a 7850-7870 video card. you will get way more performance and more value than getting another weak card to throw in your system
I don't look down on that card unlike "TheBigTroll" as nvidia's low end can't even compete with this card lol. I suggest that you look at a 7770 if you are wanting to be cheap but also not have to worry about the power supply. If you want something much more powerful then I would go for a 78x0 series. Don't get to excited about the GTX6xx series as there are some quality and software issues at this time with those cards.
HD 7770 would be a trivial upgrade. Plus, money inefficient.
As for going SLI: if you can get another card cheap (like below $50), it's a really good idea to boost your system gaming performance. In SLI the performance will be close to GTX 560, which costs $160-170 today.