Hi Gary,
Neither SketchUp nor Lumion can make use of SLI setup of video cards. For SU only, any medium decent card between the 512 Mb and 1 Gb range will do the job. SU is als a single core application so get a CPU which has as fast clock speed on a single core as possible (your setup looks okay in this respect).
Now as Lumion cannot use two cards either (it works but is not any better), the best is to get a really strong card here. nvidia gtx 580 with 3 Gb or the newest GTX 680 with 4 Gb of RAM seem to be the top at the moment (GTX 590 and 690 are actually two, smaller cards together so this is not the best)