A great site to help in your questions is
http://www.vistax64.com/ this is Vista 64 so it applies to your machine. I run Phenom X4 9850. 8 gig DDR 800 and 9800gt. I get a 5.9 Vista experience. Vista runs very well and is quick. Please note I was getting a 5.9 with just 4 gig so you do not need 8 gig to get a 5.9. You have plenty of RAM. I did read in the Vista forums that a member set up 3 accounts. Admin and 2 user accounts. She said the admin account was much slower than the user account. MS recommends you have a user account for everyone. The admin account is just for maintenance. She created a user account for herself and now Vista was working great for her. However, in my case, both accounts run the same speed for me. So who knows. Anyway, I thought it was stupid to have a user account because only I use this computer. I did not want to bounce back and forth, yadda, yadda....I find that I don't. I find that I do not need to be in the admin account. I have not been there in over a week. I can do all in user account. I just get the pop up "did you start this process?" If yes and if no...yadda. No problems. Anyway, on the Vista forum there is one forum for performance and perhaps you can start tweaking.