the EVGA GTS 450 is a solid choice. The cheapest version goes for 80 bucks after MIR from tigerdirect.com, which currently has the lowest price.
The only difference between the pricier versions is a simple OC that you could probably do yourself.
My recommendation is to get a GTS 450 now (it should run most games on max settings). And when the time comes you could get another GTS 450. The Nvidia 400 series has one of the highest performance increases from running 2 graphics cards out there. You may need to upgrade to a 600w PSU but unless your running 2 hard drives or anything like that. Your 550w, I'm almost positive can handle it.
Im also assuming that you have a monitor with a resolution of 1080p, if you have anything lower then you should have no issues.
Another thing you may want to take into consideration is that your MOBO might be toware if you have 2 graphics cards then each PCIe slot only runs at a "8x, 8x" or "16x, 8x" speed as when you have a single it runs at max 16x speed. So unless your MOBO upholds a "16x, 16x" speed during SLI, then a single 1gig GTX 260 is your best bet.