I think there is a combination of things going on here that could lead to a bottleneck. A higher amount of VRAM is almost a scheme these days; most games divert most of their memory needs to RAM or a pagefile, depending on how often they are accessed during game play. Increasing the amount of VRAM will simply not do much good. What you could do is go through a tutorial on how to optimize nvidia control panel. There are a lot of aliasing effects that can cause a bottleneck in performance. Also, I truly cannot see a difference at 1080i resolution between some settings. You can tone down some things that simply just are not important; shadows for instance. Also, your HDD may be contributing to the bottleneck. If it is a slower HDD, then the pagefile mentioned above may be slowing down read/write times. 8 gb of RAM, though a lot, can get eaten up fast if you have other programs running in the background while gaming, forcing windows to use the pagefile.
Obviously, an SSD is an expensive upgrade, but after doing some tweaking myself, I have an Intel 520 series SSD and a GTX 560 SE, which is not a super high end card in the grand scheme of things, but I can play Battlefield 3 on high settings with pretty decent FPS, certainly in the mid 20's range consistently.
Just my two cents.