Graphics card for 650W PSU

I built a PC for work and home theatre and so it doesn't have much of a graphics card, but I'm wondering how much graphics I could fit in my system without replacing my 650W power supply.

Motherboard: ASUS P8Z68-V PRO
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Processor: Intel Core i5 2500K
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Power Supply: Corsair Professional Series 650W Modular PSU
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Memory: G.Skill Ripjaws Series 16GB (4 x 4GB) 1600
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Boot Drive: OCZ Agility 3 60GB SSD
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Mass Storage: 2x Seagate Barracuda Green 2TB HDD
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I also have six case fans. There's no overclock but I'd like to keep the option open. My motherboard does support x8/x8 SLI and crossfire but I figure one card is probably less draw than two.

EDIT: Sorry, guys, I didn't follow some of the rules for asking for new GPU advice.

Purchase Date: Within the next...three months.

Budget: I don't have a budget. I'm more curious about what's realistically within reach if I ever decide to upgrade.

Usage: Gaming. I don't have a specific kind of game I enjoy.

Preferred Website: newegg.com

Overclocking: Not currently

SLI/Crossfire: don't care

Monitor Resolution: 1920x1080

Country: USA

Parts Preference: I'd like eyefinity but if an Nvidia card is clearly my best choice then that would be fine.