Going "all the way" would provide the most noticeable boost in performance for you. If you're on a budget though, you can try just the GPU at first and see whether or not you're happy with the results.
From the chart http://www.tomshardware.com/forum/forum2.php?config=tom... it looks to me that you will be getting about double the performance alone in just the GPU. That is in a good title of a game too. I would stick with the GPU upgrade and wait on all the rest... that is what I did anyhow. I just upgraded my GPU and bought a used proc, mobo, and memory for $100. But it seems to me that you already have a good system... is that opty a dual core? I'm assuming it is... I dont know much about opty's other than they are super o'cable. I would stick with what you have for now and play hard for a year than do a full upgrade. That is what I am doing anyhow... All or nothing for me... I wont even carry foward my 8800GT to the next build.
I think that is what I will do. I'll just OC the opty as far as it will go and wait. Yes it is a dual core, got it last year. That is the main reason why I hesitate because I just got it a year ago. The 7800gt, however, is showing its age.