Ok man... Just to set a few things straight, a higher power output PSU isn't going to fry your computer by plugging it in... The wattage rating on a PSU is there to indicate to you how many devices it can support/load it can handle (ie: hard disks, DVD/Blu Ray trays, Graphics cards) NOT how much power it's pumping into your system... Your PSU regulates and distributes that to the rest of your system as needed... If you got 5 hard disk drives, it'll pull more power than say, 2 hard disk drives... It's a common misconception, I've been there myself :)
Using my computer as an example, I'm STILL using a Core2Duo 4400 with 2GB of RAM right now (yes, it is due for an upgrade)... I had to upgrade my GPU a year ago cos my old one died from age, and went with the Nvidia GeForce GTS 250... That meant I had to upgrade my PSU (All PCI2 cards require a dedicated power source)...
I got a 550W PSU at the time but I went with a cheaper brand and paid for it when it fried itself out, but that's beside the point... Anyways, last week, I replaced it with a 600W CoolerMaster one (for posterity)... And it's still running everything fine, including the SWTOR Beta, no issues at all...
From the specs I see you have for your computer, the Dell Inspiron 546, it's a AMD X4 Quad Core processor... That means, Dell basically shafted you and gave you a cheaper, underpowered PSU for your system... You'll definitely want to have a better graphics processor because since I don't see any listed in your specs, that means Dell double shafted you with an integrated graphics processor (one that's hard wired into your motherboard)...
So, I say all that to say, if my waaaaay older system can handle a 600W PSU with no issues, I don't see why your's can't:)
Just go with a reliable PSU or you may end up paying the price difference eventually when it fails (I was recommended CoolerMaster or Andyson)...
Now, as for selecting a GPU, go for one that fits your budget and how long you intend to wait to upgrade it next... Something in the $150-$200 would last you 2 years, $200-250 range, 3 years, and so on (I would suggest Nvidia, since there's issues with the ATI as kindly mentioned by Zeddikas)...
Then, what you will need to do, and ask someone to help you with this if you aren't sure, is get a screwdriver, power your system down completely, pull out the power cable from the back of it, crack that baby open, yank out that old power supply, along with all the wires connected to your hard disks, DVD drives, motherboard and whatnot, put in the new one and look for a slot to plug in your new card (it'll only fit in one slot... *snark*)...
Then you have the fun job of reconnecting everything you pulled out, screw the case back together, pop in the back socket, plug your monitor into your shiny new card and then download new drivers for your card once your computer's up and running and you're ready to roll...
Hope something in that wall of text has been helpful :)