The question of whether you need a new power supply is very dependent on how good of a power supply it is. Open up your case and tell me what brand/model it is.
SouTerror - Crossfire is for ATI video cards and SLI is for NVidia. He stated he wants to stick with NVidia. Also running dual cards is pointless unless you have a good one to start with. A 9800GT will beat two 9500's for example. Also, running SLI will definitely require a more powerful power supply and I can guarantee his motherboard doesn't support it if it came with 5 GB of ram LOL
Also, running 8gb of RAM is pointless at this time. 5gb is plenty for any task that you would be doing.
Tomshardware has some good benchmarks. Here is a link to a list of NVidia cards and their performance at a lot of games.
Charts, benchmarks Gaming Graphics Cards Charts 2009 (High Quality, Update 3), Sum of FPS Benchmarks 1680x1050
Yours is equivalent to the bottom two...
The most important thing you did not mention is your budget. How much are you looking to spend with these upgrades? Your processor is fine, RAM is fine, and your power supply might work. Getting a much better video card and a better power supply (if needed) can easily be had for ~$200 or less.