If you are wanting to buy hardware I think the most bang for your buck is getting a second hard drive and putting 4 of your 5 box on it. Here is a post of someone that bought a second drive Adding second hard drive made a world of difference.

Here is what you should do to optimize your Vista. First go to http://www.blackviper.com/WinVista/servicecfg.htm and use his safe config settings if you aren't too familiar with Windows services. I used his tweaked settings with no problem, but you might want to play it safe. Second use the graphics macros from wow in-game graphic detail macros (helps performance) I use the medium macro and never have lag.

Goodluck