Okay, hold your horses. First off, we need to know what type of monitor you have. Size and max resolution specifically - this makes a huge difference.
I personally have an ATI 4870 1gig. I run a 24" monitor at 1920x1200 resolution. I need a gig of ram for that many pixels. If you're running at around 1600x1024 resolution or lower, you're probably better off with a 512mb video card (there would be zero difference between the 512 and the 1 gig versions for you). Just because a video card performs better in benchmarks doesn't mean it's right for you! A 4870X2 is a waste of money because you will get very little bang for your buck. After about ~$220, the amount of performance per dollar you get drops drastically (my personal experience is that $150-$200 is the sweet spot). Unless you have a 30" monitor, you are lighting your money on fire and watching it burn.
Also, 550 watts is fine for your system as long as you don't overclock anything and your PSU is not some crap brand (although it still may be alright). PSUs perform very, very differently; just because it says it puts out 550 watts does not necessarily mean it will perform at the level for any reasonable period of time.
I'm guessing you bought something like a Dell or Gateway?
