Assuming your power supply can actually provide 550 watts, hard to say without knowing what type it is, there are a number of options at that price point. Best bang for your buck right now would probably be the Radeon HD 5850, which is often selling for around 150 USD, as retailers are clearing out their old stock. I think the GTX 460 768MB also sits within your price range.
Either of these cards should be able to max out Rift, unless you are running a monitor at 2560x1600 or gaming across multiple monitors, in which case to max out games you would have to go beyond your price range.
If you are running a monitor at less than 1080p, the Radeon HD 5770 could probably handle your needs quite well, and would be cheaper than the two previously mentioned cards.