Re: Upgrading my computer for cost effectiveness, please help
First of all, the whole "I'm bad at this" is redundant. If you're posting here, you're asking for help, that's cool. No need to linger on the thought, there are definitely tons of folks who don't have as much experience as some, and that's ok.
The Ti 550 is a good card, and while your current system can run it, you're cutting it a little close with the power supply. If you can wing it, try upgrading to a 600 watt unit that will give you some breathing room. Even one of Antec's 520 watt units should be fine as well. For 600 watts I would go with either Seasonic, Thermaltake or Corsair. Don't skimp on a power supply. If you find a "600 watt" unit for under $50, it's a joke, and you'll put your hardware at serious risk.
Swapping a video card out is cake - disconnect the power leads going to it (if any) unscrew the mounting bracket (or un clip it, if it's a clip) carefully pull the tab on the board (this part varies by board maker) and then carefully lift the video card straight out of the slot.
Power supplies are even easier, just make note of what is plugged in and where, and unplug them, install the new unit, plug in the same leads, they only go in one way, and presto, all done.