@Nayl: You won't have to have a particularly amazing rig to run Rift on even high settings as it seems to be optimized pretty freakin' well, from all I've seen.
I don't know what the rest of your computer is like, so posting information like the particular model and wattage on your power supply unit (if you know how; I'm assuming you do, and if not, we can help you there too!) will help us figure out what kind of graphics card you can/should pick up, as well as the ideal price you'd want to spend on a graphics card, will all help. Pretty much, just throw all the information you can at us; it'd be nice to know the specifics of your processor and RAM, as well, so you can avoid bottlenecking your newly-found graphical horsepower. Any information you can provide is important. Anything helps, like what kind of connectors your PSU totes to if your motherboard has PCI-Express slots, so tell us all you can!
tl;dr: if you can get us the approximate top end of moolah you want to spend on a graphics card and tell us all you know about your current setup, I'm sure the tech-heads here can find you the perfect card.
Just off the top of my head, I use a single GTX 460 and while I did tamper with the speed a bit, it runs Rift with every single setting maxed - in windowed mode - on a dual-core processor with 4GB RAM. You can also snag what is one of (if not arguably...) THE best GPU brands, EVGA, to pair with one of the greatest budget beast cards for all of $125 on sale, sometimes even less after rebates. Of course, depending on what the rest of your setup and your budget is like, there may be better deals. Keep us updated!