3-boxing Rift Which Video Card HD5830 Or GTX 550 TI Superclocked?

[tripleboxed]

I had done some research on which video card is right for a person like me trying to 3-box Rift.

I have now an idea on which might be best basicly the HD5830 At tigerdirect.com for 110$ -125$USD Powerful card with 1GB of DDR5 Memory clock of 800.

They say nvida is better then ati becuase ATI is old & now im looking at the GTX 550 TI Superclocked for 134.99*
what i dont understand is why the GTX 550 More when the HD5830 is cheaper & better perfomance wise ill have to go with the GTX 550 TI for being overclocked and having 1080p HD support what do you guys think i should go for at a price range of 135$

My Custom Built Rig Specs Built 2010
CPU Model AMD Athlon 64 X4se 2.1GHz Video Card: HD 4200 Onborad - runs rift smoothly on low quietly render during warfronts & Instaces Motherboard: ASUS M4A785-M AMD 785G Ram: G.SKILL 4GB Duel Channel DDR2 Storage: 200 Soundcard & Speakers: Onborad Power Supply 650 Watt Ultra Temp