GTX 280 Question

My 8800GT is getting a little old for my tastes. I'm running a Dell with an upgraded 550W Antec PSU, so my options are somewhat limited. I'm really excited about the new i7 processors and emergence of Tri-Channel DDR3, but I'd like to wait for the market to settle down and for these companies to iron out any kinks in their new products before I dive in and build my first computer. I've basically decided I want a GTX 280 now to bridge the gap between now and my new rig, and when I build the new rig I'll supplement it with another GTX 280 (or two). If it matters for this thread, I game at 1680x1050 for now and I might step up to a 24" monitor in the future (perhaps even larger if prices keep dropping).

Here are the questions:

An EVGA-built GTX 280 would be the best choice for me because of the Step Up program, right? I mean, I'm not entirely sure that I will want a GTX 280 in a couple months and I don't want to be stuck with this decision if something better comes out when I build my new rig. I know it's a strange question but I want to limit my potential regrets, which I did not do when I bought my 8800GT (Radeon 4000 series came out a couple months later for roughly the same price as what I paid).

I'm also curious as to whether or not my current PSU could even handle a GTX 280. Power requirements state: "Minimum of a 550 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 40 Amp Amps)." Here is a list of my PSU's outputs: +3.3V@28A,+5V@26A,+12V1@22A,+12V2@22A, +12V3@25A,-12V@0.5A,+5VSB@2.5A. Do I add up the Amp rating from each +12V rail to decide if I can power this card? If so, that would be 69A. I'm sure it's not that simple, but Google was not helping me figure this one out. Here's a link to my PSU (I got it for $60 about 6 months ago):
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Before I get a bunch of fanboyisms, I give the 4870x2 zero consideration simply because of its high power demand. I'm aware that it's a better card overall, but I need a solid, powerful card that I can squeeze into my current machine and still maintain some kind of strength down the road.