So you're running skyrim on high with 768 MB's of Vram? What resolution and fps?
What resolution can I expect to play on with 1 gig of vram and the i5 cpu?
The i5 is a good CPU, I should know as I have it. No need for overclocking, it's plenty powerful.
VRAM doesn;t really matter so much as the clock speeds of the GPU. Most modern GPUs with 1024 MB of VRAM should be plenty fast to play it though. What it comes down to is whether or not you want an Nvidia or ATI card. They both have good cards so it seems to come down to personal preference more often than not. I use Nvidia, but that's less to do with them being better then ATI and rather with the fact that I've always had Nvidia.
And of course when building a computer there are other things to think about. How much RAM do I want? Will I be getting a 32x or 64x OS to use all of my RAM? How powerful should my PSU be? What motherboard will I get? What size of a case will I be needing to house the motherboard? And others.
As for my personal FPS, I get about a 50 FPS average around the game running with the intel i5 2500K, a Nvidia GeForce GTX 460, and 4 GB of RAM at a resolution of 1920x1080. Except in Markarth. Freaking. Markarth. There it drops to about 35.