Video card for dual WQHD monitors

[wikksmith]

Recently added 2 WQHD monitors (ASUS PB278Q) to an old system (see below).
Contrary to the salesman's prediction, everything works well at maximum resolution (2560x1440) on both monitors, except for transient, minor stuttering on SOME over the air programming. No problems with analog cable video. Over the air channel strength is very good according to Windows Media Center utility. System is used for Photoshop, video editing (Sony Vegas), audio editing (Sony Acid), and TV. No gaming.
Questions 1: what is the biggest bottleneck, GPU? VRAM? CPU?
Question 2: if it's the video card, what are some reasonable options? I have a GTX 570 (1.2 GB) in another system but would rather not bother with a swap if a 570 is not likely to improve HD video.
Thanks!

Win7 64-bit, EVGA GTX260 Core 216 (626MHz, 0.898 GB), Gigabyte GA-EP45-UD3P, Intel Q9550 (2.8GHz), 8GB RAM (G.Skill 6400), Hauppauge HVR-1600 tuner card, Corsair 750W.

 

[lowriderflow]

I'm not familiar with the video editing world... if you were gaming, you'd be short on both, power and VRAM.

a 570 is like double the performance of a 460, so that'd defintely help!

my recomendation, install MSI afterburner and use hte monitoring built in.
you can monitor GPU % usage, GPU VRAM usage, system RAM usage, and CPU % usage. so you can see in real time which thing is hitting 100% and bottlenecking

 

[wikksmith]

Lowrider:
Thanks for the suggestion. I've loaded Afterburner. With both monitors at full screen, max resolution, and Windows Media Center with over the air HD channel on one, Internet explorer with video on the other, the gtx260 is using less than 600MB of VRAM. I'm still learning how to use afterburner-didn't see the GPU usage %.