Best card supported by my Asus P6T motherboard

Sooner is better, e.g. within a half week to a week but it's not critical. Assume no budget limit. I would like a card that could allow my computer to last a couple more years.

I'm going to be using it for gaming, primarily MMOs, right now the name of the game is guild wars 2 and my current card (GTX275) can't handle large scale world vs world battles but I'd also like a card capable of running a game like Cryisis close to maxed out if possible.

GTX 275, Corsair TX 750 W power supply

Intel Core i7 CPU 920 @ 2.67GHz, Asus P6T motherboard, 12 GBs RAM (DDR3?-Unsure), no idea as to the case/cooling other than that it has an Antec logo on it.

No real preference on retailers, I live in western Canada, British Columbia.

Whatever is compatible and reliable will do, I've only ever used Nvidia cards.

I doubt I will overclock.

1920x1080 triple monitor setup, though I'd be willing to sacrifice one monitor for a card with better performance than something with triple monitor support.

Basically I just want a card that can deal with the huge numbers of players and the vast number of particle effects being thrown around in a large scale battles inherent to MMOs and maintain at least 50fps. Something that vastly improves upon my 10-12 fps in aforementioned scenarios. Thanks.