Sounds similar to some results I found when clocking my 6950 - I found upping the GPU speed rarely impacted the results and concluded the CPU was holding back the card in the benchmarks. I also saw decreases when using a RAMDisk - I think (but am no techie) that the CPU/GPU get the data faster and so have more processing to perform, hence the slight decrease in fps. When running from a slower disk the system processes what it can with the data to hand so fps goes up, but the you see texture pop ups when the data finally arrives and is added. I should add I am using fancycache these days as all the files are on SSD disks so there not much in it for a RAMdisk.
@Bangtail thanks for the info - I tracked VRAM use on the card and have seen it push 1.985Gb so believe the headroom of over 2Gb is a worthwhile increase and will also wait to see the next raft of Nvidia cards. Hopefully the multi screen feature will be fully understood then as I currently use Eyefinity and it works very well.
Incidently, I fixed a recent period of stuttering in the game which I had put down to either hitting the 2Gb VRAM limit or a disk/memory issue, but found no solution with either a RAMdisk, fancycache or reduction in detail/quality. The solution was my Crucial SSD, which had a firmware update to cure disk access stuttering which was passed on to the game. A quick firmware update to the latest version fixed the stuttering in a fhash (no pun intended). Daft thing was the stuttering was only on one or MP missions.