I recommend getting another power supply before upgrading your video card. Many cheap power supplies don't put out their stated wattage. Does it have a gpu six pin power connector? What are the specs of the rest of your system?
doing a little math and some guessing:
adding up those rails 3.3, 5 and 12 volt) gives you the total of ~580 watts (i think that is what i got but i cleared my calculator) which is ~128% of 450 watts; in other words 28% more. so even though each rail may be rated at those amps that is not what it can supply combined.
so removing 28% of 12V2 (which the pci connection would be on) (12*15)/1.28=140.625 watts. keeping that at 80% 140.625*.8= 112.5 watts. chances are your sata power is sharing the same rail so remove 10 watts for the hard drives. (spinning up they'll use 10 each but will use half that after boot up)
102 watts rounded down. without knowing the actually quality of the PSU; that is the best i would expect to be available for a graphics card.
now those are stock non overclocked reference cards. you are talking about a 5770/7770/650ti range to be relatively comfortable. maybe a 7850/6850/450
but that is selling your i3 2100 a little short; getting a better PSU, even a 450 watt one that is better quality will get you a 7850 easily and the prospect of a 660/7870 card.
If you want to spend some more money and get one that will handle future upgrades or a better card right away this is a good way to go: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I believe that my PSU, XFX core edition, is pretty much the same thing mine just has more power (like I said, buying more than you need for future upgrades is a good thing) the point being that I like my PSU and it's been good so far.