What your describing sounds more like video card performance issues than anything to do with memory or CPU.  And all major OpenGL apps will have major slow down if you pile on enough polygons, it's not just studio.  GTS450's aren't that bad, the drivers are at least decent.  But the difference between my old GTX480 and the new GTX680 was night and day.  I played with a Titan for a few hours (it was a loaner from nVidia) and to be honest I don't think it was worth the premium.  But I haven't used any of the 700's yet they may be a better call than my 680.

I'd try changing the settings in DS for the OpenGL.  Move the slider to performance.  Set a low limit on texture size.  Turn off antialiasing.  And change your viewport to bounding box or wireframe mode.  When working with large scenes you may find you can't do the everything is a fully texture shaded mode.

If you can make DS crash and it generates a crash report, save it out and go report it on DAZ's Mantis bug tracker https://bugs.daz3d.com  You will need to create a new account there (it doesn't use your DAZ account).  A crash dump can be very helpful.  I don't think the submit thing works but you can grab the zip and including in your bug report.

-Matt