My CS5/AVCHD 1st Impressions http://forums.adobe.com/thread/652694?tstart=0 includes a link to the computer I built... since the GTX 285 is no longer sold, I would now go with a GTX 460 or whatever is the current "best bang for the buck" - AND I would buy 4Gig-by-3Sticks memory to be able to expand from 12Gig to 24Gig if needed
For my home hobbyist, family movies (which means that I am not trying to recreate Star Wars with video effects or many layers) AVCHD editing is "as smooth as spreading warm butter on hot toast" (also the MP4 video from wife's Flip camera)
My 3 hard drives are configured as... (WD = Western Digital)
1 - 320G WD Win7 64bit Pro and all programs
2 - 320G WD Win7 swap file and video projects
3 - 1T WD all video files... read and write
I do not know what motherboard you have so I cannot precisely answer your question. If you went to the PPBM5 site you may have noticed that generally the best results are with over 12 GB. There seems to be a step function increase in performance.
I am guessing now but if you have a six memory slot motherboard and if two of those slots are currently in use, I would suggest you get identical specification modules and get a set of 3 each 4 GB versions so you would end up with 16 GB. Install the new modules in the first set of slots and the original two modules in the second memory channel. But notice your current memory is ECC memory and you have to get slight more expensive new ECC modules (as opposed to non-ECC pricing) as generally required for Xeon systems.